Adaptive forward power control and adaptive reverse power control for spread-spectrum communications

ABSTRACT

A wireless code division multiple access (CDMA) communication system, including a base station (BS) and a subscriber unit (SU), uses adaptive forward power control (AFPC) and adaptive reverse power control (ARPC) to control the transmit power of forward and reverse link radio frequency (RF) channel signals, respectively. Automatic gain control (AGC) signals are produced to maintain the level of forward and reverse CDMA signals at a near constant level. The CDMA signals are despread to produce first and second despread signals. The first and second despread signals are used to produce a first error signal e 1 . A second error signal e 2  is produced based on the AGC signals. Combined error signals are formed based on the first error signal e 1  and the second error signal e 2 . The combined error signals are hard limited to form adaptive power control (APC) bits used to control the level of the forward and reverse link RF channel signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part to U.S. patent applicationSer. No. 08/956,740 filed on Oct. 23, 1997, which issued on Apr. 10,2001 as U.S. Pat. No. 6,215,778; which is a division of U.S. patentapplication Ser. No. 08/669,775 filed on Jun. 27, 1996, which issued onAug. 25, 1998 as U.S. Pat. No. 5,799,010; which claims the benefit ofU.S. Provisional Application No. 60/000,775 filed on Jun. 30, 1995.

FIELD OF INVENTION

The present invention generally pertains to Code Division MultipleAccess (CDMA) communications, also known as spread-spectrumcommunications. More particularly, the present invention pertains to asystem and method for providing a high capacity, CDMA communicationssystem which provides for one or more simultaneous user bearer channelsover a given radio frequency, allowing dynamic allocation of bearerchannel rate while rejecting multipath interference.

BACKGROUND

Providing quality telecommunication services to user groups which areclassified as remote, such as rural telephone systems and telephonesystems in underdeveloped countries, has proven to be a challenge inrecent years. These needs have been partially satisfied by wirelessradio services, such as fixed or mobile frequency division multiplex(FDM) systems, frequency division multiple access (FDMA) systems, timedivision multiplex (TDM) systems, time division multiple access (TDMA)systems, combination frequency and time division (FD/TDMA) systems, andother land mobile radio systems. Usually, these remote services arefaced with more potential users than can be supported simultaneously bytheir frequency or spectral bandwidth capacity.

Recognizing these limitations, recent advances in wirelesscommunications have used spread spectrum modulation techniques toprovide simultaneous communication by multiple users. Spread spectrummodulation refers to modulating an information signal with a spreadingcode signal; the spreading code signal being generated by a codegenerator where the period Tc of the spreading code is substantiallyless than the period of the information data bit or symbol signal. Thecode may modulate the carrier frequency upon which the information hasbeen sent, called frequency-hopped spreading, or may directly modulatethe signal by multiplying the spreading code with the information datasignal, called direct-sequence (DS) spreading. Spread-spectrummodulation produces a signal with bandwidth substantially greater thanthat required to transmit the information signal. Synchronous receptionand despreading of the signal at the receiver recovers the originalinformation. A synchronous demodulator in the receiver uses a referencesignal to synchronize the despreading circuits to the inputspread-spectrum modulated signal to recover the carrier and informationsignals. The reference signal can be a spreading code which is notmodulated by an information signal. Such use of a synchronousspread-spectrum modulation and demodulation for wireless communicationis described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUSSPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling,which techniques are incorporated herein by reference.

Spread-spectrum modulation in wireless networks offers many advantagesbecause multiple users may use the same frequency band with minimalinterference to each user's receiver. Spread-spectrum modulation alsoreduces effects from other sources of interference. In addition,synchronous spread-spectrum modulation and demodulation techniques maybe expanded by providing multiple message channels for a single user,each spread with a different spreading code, while still transmittingonly a single reference signal to the user. Such use of multiple messagechannels modulated by a family of spreading codes synchronized to apilot spreading code for wireless communication is described in U.S.Pat. No. 5,166,951 entitled HIGH CAPACITY SPREAD-SPECTRUM CHANNEL byDonald L. Schilling, which is incorporated herein by reference.

One area in which spread-spectrum techniques are used is in the field ofmobile cellular communications to provide personal communicationservices (PCS). Such systems desirably support large numbers of users,control Doppler shift and fade, and provide high speed digital datasignals with low bit error rates. These systems employ a family oforthogonal or quasi-orthogonal spreading codes, with a pilot spreadingcode sequence synchronized to the family of codes. Each user is assignedone of the spreading codes as a spreading function. Related problems ofsuch a system are: supporting a large number of users with theorthogonal codes, handling reduced power available to remote units, andhandling multipath fading effects. Solutions to such problems includeusing phased-array antennas to generate multiple steerable beams andusing very long orthogonal or quasi-orthogonal code sequences. Thesesequences may be reused by cyclic shifting of the code synchronized to acentral reference and diversity combining of multipath signals. Suchproblems associated with spread spectrum communications, and methods toincrease the capacity of a multiple access, spread-spectrum system aredescribed in U.S. Pat. No. 4,901,307 entitled SPREAD SPECTRUM MULTIPLEACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS byGilhousen et al. which is incorporated herein by reference.

The problems associated with the prior art systems focus around reliablereception and synchronization of the receiver despreading circuits tothe received signal. The presence of multipath fading introduces aparticular problem with spread spectrum receivers in that a receivermust somehow track the multipath components to maintain code-phase lockof the receiver's despreading means with the input signal. Prior artreceivers generally track only one or two of the multipath signals, butthis method is not satisfactory because the combined group of low powermultipath signal components may actually contain far more power than theone or two strongest multipath components. The prior art receivers trackand combine the strongest components to maintain a predetermined biterror rate (BER) of the receiver. Such a receiver is described, forexample, in U.S. Pat. No. 5,109,390 entitled DIVERSITY RECEIVER IN ACDMA CELLULAR TELEPHONE SYSTEM by Gilhousen et al. A receiver thatcombines all multipath components, however, is able to maintain thedesired BER with a signal power that is lower than that of prior artsystems because more signal power is available to the receiver.Consequently, there is a need for a spread spectrum communication systememploying a receiver that tracks substantially all of the multipathsignal components, so that substantially all multipath signals may becombined in the receiver, and hence the required transmit power of thesignal for a given BER may be reduced.

Another problem associated with multiple access, spread-spectrumcommunication systems is the need to reduce the total transmitted powerof users in the system, since users may have limited available power. Anassociated problem requiring power control in spread-spectrum systems isrelated to the inherent characteristic of spread-spectrum systems thatone user's spread-spectrum signal is received by another user's receiveras noise with a certain power level. Consequently, users transmittingwith high levels of signal power may interfere with other users'reception. Also, if a user moves relative to another user's geographiclocation, signal fading and distortion require that the users adjusttheir transmit power level to maintain a particular signal quality. Atthe same time, the system should keep the power that the base stationreceives from all users relatively constant. Finally, because it ispossible for the spread-spectrum system to have more remote users thancan be supported simultaneously, the power control system should alsoemploy a capacity management method which rejects additional users whenthe maximum system power level is reached.

Prior spread-spectrum systems have employed a base station that measuresa received signal and sends an adaptive power control (APC) signal tothe remote users. Remote users include a transmitter with an automaticgain control (AGC) circuit which responds to the APC signal. In suchsystems the base station monitors the overall system power or the powerreceived from each user, and sets the APC signal accordingly. Such aspread-spectrum power control system and method is described in U.S.Pat. No. 5,299,226 entitled ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUMCOMMUNICATION SYSTEM AND METHOD, and U.S. Pat. No. 5,093,840 entitledADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUM TRANSMITTER, both by DonaldL. Schilling and incorporated herein by reference. This open loop systemperformance may be improved by including a measurement of the signalpower received by the remote user from the base station, andtransmitting an APC signal back to the base station to effectuate aclosed loop power control method. Such closed loop power control isdescribed, for example, in U.S. Pat. No. 5,107,225 entitled HIGH DYNAMICRANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT to Charles E. Wheatley,III et al. and incorporated herein by reference.

These power control systems, however, exhibit several disadvantages.First, the base station must perform complex power control algorithms,increasing the amount of processing in the base station. Second, thesystem actually experiences several types of power variation: variationin the noise power caused by the variation in the number of users andvariations in the received signal power of a particular bearer channel.These variations occur with different frequency, so simple power controlalgorithms can be optimized to compensate for only one of the two typesof variation. Finally, these power algorithms tend to drive the overallsystem power to a relatively high level. Consequently, there is a needfor a spread-spectrum power control method that rapidly responds tochanges in bearer channel power levels, while simultaneously makingadjustments to all users' transmit power in response to changes in thenumber of users. Also, there is a need for an improved spread-spectrumcommunication system employing a closed loop power control system whichminimizes the system's overall power requirements while maintaining asufficient BER at the individual remote receivers. In addition, such asystem should control the initial transmit power level of a remote userand manage total system capacity.

Spread-spectrum communication systems desirably should support largenumbers of users, each of which has at least one communication channel.In addition, such a system should provide multiple generic informationchannels to broadcast information to all users and to enable users togain access to the system. Using prior art spread-spectrum systems thiscould only be accomplished by generating large numbers of spreading codesequences.

Further, spread-spectrum systems should use sequences that areorthogonal or nearly orthogonal to reduce the probability that areceiver locks to the wrong spreading code sequence or phase. The use ofsuch orthogonal codes and the benefits arising therefrom are outlined inU.S. Pat. No. 5,103,459 entitled SYSTEM AND METHOD FOR GENERATING SIGNALWAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM, by Gilhousen et al. andU.S. Pat. No. 5,193,094 entitled METHOD AND APPARATUS FOR GENERATINGSUPER-ORTHOGONAL CONVOLUTIONAL CODES AND THE DECODING THEREOF, by AndrewJ. Viterbi, both of which are incorporated herein by reference. However,generating such large families of code sequences with such properties isdifficult. Also, generating large code families requires generatingsequences which have a long period before repetition. Consequently, thetime a receiver takes to achieve synchronization with such a longsequence is increased. Prior art spreading code generators often combineshorter sequences to make longer sequences, but such sequences may nolonger be sufficiently orthogonal. Therefore, there is a need for animproved method for reliably generating large families of code sequencesthat exhibit nearly orthogonal characteristics and have a long periodbefore repetition, but also include the benefit of a short code sequencethat reduces the time to acquire and lock the receiver to the correctcode phase. In addition, the code generation method should allowgeneration of codes with any period, since the spreading code period isoften determined by parameters used such as data rate or frame size.

Another desirable characteristic of spreading code sequences is that thetransition of the user data values occur at a transition of the codesequence values. Since data typically has a period which is divisible by2^(N), such a characteristic usually requires the code-sequence to be aneven length of 2^(N). However, code generators, as is well known in theart, generally use linear feedback shift registers which generate codesof length 2^(N)−1. Some generators include a method to augment thegenerated code sequence by inserting an additional code value, asdescribed, for example, in U.S. Pat. No. 5,228,054 entitled POWER-OF-TWOLENGTH PSEUDONOISE SEQUENCE GENERATOR WITH FAST OFFSET ADJUSTMENT byTimothy Rueth et al. and incorporated herein by reference. Consequently,the spread-spectrum communication system should also generate spreadingcode sequences of even length.

Finally, the spread-spectrum communication system should be able tohandle many different types of data, such as FAX, voiceband data andISDN, in addition to traditional voice traffic. To increase the numberof users supported, many systems employ encoding techniques such asADPCM to achieve “compression” of the digital telephone signal. FAX,ISDN and other data, however, require the channel to be a clear channel.Consequently, there is a need for a spread spectrum communication systemthat supports compression techniques that also dynamically modify thespread spectrum bearer channel between an encoded channel and a clearchannel in response to the type of information contained in the user'ssignal.

SUMMARY

The present invention is embodied in a multiple access, spread spectrumcommunication system which processes a plurality of information signalsreceived simultaneously over telecommunication lines for simultaneoustransmission over a radio frequency (RF) channel as acode-division-multiplexed (CDM) signal. The system includes a radiocarrier station (RCS) which receives a call request signal thatcorresponds to a telecommunication line information signal, and a useridentification signal that identifies a user to which the call requestand information signal are addressed. The receiving apparatus is coupledto a plurality of code division multiple access (CDMA) modems, one ofwhich provides a global pilot code signal and a plurality of messagecode signals, and each of the CDMA modems combines one of the pluralityof information signals with its respective message code signal toprovide a spread-spectrum processed signal. The plurality of messagecode signals of the plurality of CDMA modems are synchronized to theglobal pilot code signal. The system also includes assignment apparatusthat is responsive to a channel assignment signal for coupling therespective information signals received on the telecommunication linesto indicated ones of the plurality of modems. The assignment apparatusis coupled to a time-slot exchange means. The system further includes asystem channel controller coupled to a remote call-processor and to thetime-slot exchange means. The system channel controller is responsive tothe user identification signal, to provide the channel assignmentsignal. In the system, an RF transmitter is connected to all of themodems to combine the plurality of spread-spectrum processed messagesignals with the global pilot code signal to generate a CDM signal. TheRF transmitter also modulates a carrier signal with the CDM signal andtransmits the modulated carrier signal through an RF communicationchannel.

The transmitted CDM signal is received from the RF communication channelby a subscriber unit (SU) which processes and reconstructs thetransmitted information signal assigned to the subscriber. The SUincludes a receiving means for receiving and demodulating the CDM signalfrom the carrier. In addition, the SU comprises a subscriber unitcontroller and a CDMA modem which includes a processing means foracquiring the global pilot code and despreading the spread-spectrumprocessed signal to reconstruct the transmitted information signal.

The RCS and the SUs each contain CDMA modems for transmission andreception of telecommunication signals including information signals andconnection control signals. The CDMA modem comprises a modem transmitterhaving: a code generator for providing an associated pilot code signaland for generating a plurality of message code signals; a spreadingmeans for combining each of the information signals, with a respectiveone of the message code signals to generate spread-spectrum processedmessage signals; and a global pilot code generator which provides aglobal pilot code signal to which the message code signals aresynchronized.

The CDMA modem also comprises a modem receiver having associated pilotcode acquisition and tracking logic. The associated pilot codeacquisition logic includes an associated pilot code generator; a groupof associated pilot code correlators for correlating code-phase delayedversions of the associated pilot signal with a receive CDM signal forproducing a despread associated pilot signal. The code phase of theassociated pilot signal is changed responsive to an acquisition signalvalue until a detector indicates the presence of the despread associatedpilot code signal by changing the acquisition signal value. Theassociated pilot code signal is synchronized to the global pilot signal.The associated pilot code tracking logic adjusts the associated pilotcode signal in phase responsive to the acquisition signal so that thesignal power level of the despread associated pilot code signal ismaximized. Finally, the CDMA modem receiver includes a group of messagesignal acquisition circuits. Each message signal acquisition circuitincludes a plurality of receive message signal correlators forcorrelating one of the local receive message code signals with the CDMsignal to produce a respective despread receive message signal.

To generate large families of nearly mutually orthogonal codes used bythe CDMA modems, the present invention includes a code sequencegenerator. The code sequences are assigned to a respective logicalchannel of the spread-spectrum communication system, which includesIn-phase (I) and quadrature (Q) transmission over RF communicationchannels. One set of sequences is used as pilot sequences which are codesequences transmitted without modulation by a data signal. The codesequence generator circuit includes a long code sequence generatorincluding a linear feedback shift register, a memory which provides ashort, even code sequence, and a plurality of cyclic shift, feedforwardsections which provide other members of the code family which exhibitminimal correlation with the code sequence applied to the feedforwardcircuit. The code sequence generator further includes a group of codesequence combiners for combining each phase shifted version of the longcode sequence with the short, even code sequence to produce a group, orfamily, of nearly mutually orthogonal codes.

Further, the present invention includes several methods for efficientutilization of the spread-spectrum channels. First, the system includesa bearer channel modification system which comprises a group of messagechannels between a first transceiver and second transceiver. Each of thegroup of message channels supports a different information signaltransmission rate. The first transceiver monitors a received informationsignal to determine the type of information signal that is received, andproduces a coding signal relating to the coding signal. If a certaintype of information signal is present, the first transceiver switchestransmission from a first message channel to a second message channel tosupport the different transmission rate. The coding signal istransmitted by the first transceiver to the second transceiver, and thesecond transceiver switches to the second message channel to receive theinformation signal at a different transmission rate.

Another method to increase efficient utilization of the bearer messagechannels is the method of idle-code suppression used by the presentinvention. The spread-spectrum transceiver receives a digital datainformation signal including a predetermined flag pattern correspondingto an idle period. The method includes the steps of: 1) delaying andmonitoring the digital data signal; 2) detecting the predetermined flagpattern; 3) suspending transmission of the digital data signal when theflag pattern is detected; and 4) transmitting the data signal as aspread-spectrum signal when the flag pattern is not detected.

The present invention includes a system and method for closed loop APCfor the RCS and SUs of the spread-spectrum communication system. The SUstransmit spread-spectrum signals, the RCS acquires the spread-spectrumsignals, and the RCS detects the received power level of thespread-spectrum signals plus any interfering signal including noise. TheAPC system includes the RCS and a plurality of SUs, wherein the RCStransmits a plurality of forward channel information signals to the SUsas a plurality of forward channel spread-spectrum signals having arespective forward transmit power level, and each SU transmits to thebase station at least one reverse spread-spectrum signal having arespective reverse transmit power level and at least one reverse channelspread-spectrum signal which includes a reverse channel informationsignal.

The APC system includes an adaptive forward power control (AFPC) system,and an adaptive reverse power control (ARPC) system. The AFPC systemoperates by measuring, at the SU, a forward signal-to-noise ratio of therespective forward channel information signal, generating a respectiveforward channel error signal corresponding to a forward error betweenthe respective forward signal-to-noise ratio and a pre-determinedsignal-to-noise value, and transmitting the respective forward channelerror signal as part of a respective reverse channel information signalfrom the SU to the RCS. The RCS includes a plural number of AFPCreceivers for receiving the reverse channel information signals andextracting the forward channel error signals from the respective reversechannel information signals. The RCS also adjusts the respective forwardtransmit power level of each one of the respective forwardspread-spectrum signals responsive to the respective forward errorsignal.

The ARPC system operates by measuring, in the RCS, a reversesignal-to-noise ratio of each of the respective reverse channelinformation signals, generating a respective reverse channel errorsignal representing an error between the respective reverse channelsignal-to-noise ratio and a respective pre-determined signal-to-noisevalue, and transmitting the respective reverse channel error signal as apart of a respective forward channel information signal to the SU. EachSU includes an ARPC receiver for receiving the forward channelinformation signal and extracting the respective reverse error signalfrom the forward channel information signal. The SU adjusts the reversetransmit power level of the respective reverse spread-spectrum signalresponsive to the respective reverse error signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a block diagram of a code division multiple accesscommunication system according to the present invention.

FIG. 2 a is a block diagram of a 36 stage linear shift register suitablefor use with long spreading code of the code generator of the presentinvention.

FIG. 2 b is a block diagram of circuitry which illustrates thefeed-forward operation of the code generator.

FIG. 2 c is a block diagram of an exemplary code generator of thepresent invention including circuitry for generating spreading codesfrom the long spreading codes and the short spreading codes.

FIG. 2 d is an alternate embodiment of the code generator circuitincluding delay elements to compensate for electrical circuit delays.

FIG. 3 a is a graph of the constellation points of the pilot spreadingcode QPSK signal.

FIG. 3 b is a graph of the constellation points of the message channelQPSK signal.

FIG. 3 c is a block diagram of exemplary circuitry which implements themethod of tracking the received spreading code phase of the presentinvention.

FIG. 4 is a block diagram of the tracking circuit that tracks the medianof the received multipath signal components.

FIG. 5 a is a block diagram of the tracking circuit that tracks thecentroid of the received multipath signal components.

FIG. 5 b is a block diagram of the Adaptive Vector Correlator.

FIG. 6 is a block diagram of exemplary circuitry which implements theacquisition decision method of the correct spreading code phase of thereceived pilot code of the present invention.

FIG. 7 is a block diagram of an exemplary pilot rake filter whichincludes the tracking circuit and digital phase locked loop fordespreading the pilot spreading code, and generator of the weightingfactors of the present invention.

FIG. 8 a is a block diagram of an exemplary adaptive vector correlatorand matched filter for despreading and combining the multipathcomponents of the present invention.

FIG. 8 b is a block diagram of an alternative implementation of theadaptive vector correlator and adaptive matched filter for despreadingand combining the multipath components of the present invention.

FIG. 8 c is a block diagram of an alternative embodiment of the adaptivevector correlator and adaptive matched filter for despreading andcombining the multipath components of the present invention.

FIG. 8 d is a block diagram of the Adaptive Matched Filter of oneembodiment of the present invention.

FIG. 9 is a block diagram of the elements of an exemplary radio carrierstation (RCS) of the present invention.

FIG. 10 is a block diagram of the elements of an exemplary multiplexersuitable for use in the RCS shown in FIG. 9.

FIG. 11 is a block diagram of the elements of an exemplary wirelessaccess controller (WAC) of the RCS shown in FIG. 9.

FIG. 12 is a block diagram of the elements of an exemplary modeminterface unit (MIU) of the RCS shown in FIG. 9.

FIG. 13 is a high level block diagram showing the transmit, receive,control and code generation circuitry of the CDMA modem.

FIG. 14 is a block diagram of the transmit section of the CDMA modem.

FIG. 15 is a block diagram of an exemplary modem input signal receiver.

FIG. 16 is a block diagram of an exemplary convolutional encoder as usedin the present invention.

FIG. 17 is a block diagram of the receive section of the CDMA modem.

FIG. 18 is a block diagram of an exemplary adaptive matched filter asused in the CDMA modem receive section.

FIG. 19 is a block diagram of an exemplary pilot rake as used in theCDMA modem receive section.

FIG. 20 is a block diagram of an exemplary auxiliary pilot rake as usedin the CDMA modem receive section.

FIG. 21 is a block diagram of an exemplary video distribution circuit(VDC) of the RCS shown in FIG. 9.

FIG. 22 is a block diagram of an exemplary RF transmitter/receiver andexemplary power amplifiers of the RCS shown in FIG. 9.

FIG. 23 is a block diagram of an exemplary SU of the present invention.

FIG. 24 is a flow-chart diagram of an exemplary call establishmentalgorithm for an incoming call request used by the present invention forestablishing a bearer channel between an RCS and an SU.

FIG. 25 is a flow-chart diagram of an exemplary call establishmentalgorithm for an outgoing call request used by the present invention forestablishing a bearer channel between an RCS and an SU.

FIG. 26 is a flow-chart diagram of an exemplary maintenance powercontrol algorithm of the present invention.

FIG. 27 is a flow-chart diagram of an exemplary AFPC algorithm of thepresent invention.

FIG. 28 is a flow-chart diagram of an exemplary ARPC algorithm of thepresent invention.

FIG. 29 is a block diagram of an exemplary closed loop power controlsystem of the present invention when the bearer channel is established.

FIG. 30 is a block diagram of an exemplary closed loop power controlsystem of the present invention during the process of establishing thebearer channel.

FIG. 31 is a schematic overview of an exemplary code division multipleaccess communication system in accordance with the present invention.

FIG. 32 is a diagram showing the operating range of a base station.

FIG. 33 is a timing diagram of communication signals between a basestation and a subscriber unit.

FIG. 34 is a flow diagram of the establishment of a communicationchannel between a base station and a subscriber unit.

FIG. 35 is a graph of the transmission power output from a subscriberunit.

FIGS. 36A and 36B are flow diagrams of the establishment of acommunication channel between a base station and a subscriber unit inaccordance with the preferred embodiment of the present invention usingshort codes.

FIG. 37 is a graph of the transmission power output from a subscriberunit using short codes.

FIG. 38 shows the adaptive selection of short codes.

FIG. 39 is a block diagram of a base station in accordance with thepresent invention.

FIG. 40 is a block diagram of an exemplary subscriber unit in accordancewith the present invention.

FIGS. 41A and 41B are flow diagrams of a ramp-up procedure implementedin accordance with the present invention.

FIG. 42 is a prior art CDMA communication system.

FIG. 43 is a graph of the distribution of acquisition opportunities ofthe system of FIG. 42.

FIG. 44 is a diagram showing the propagation of signals between a basestation and a plurality of subscriber units.

FIG. 45 is a flow diagram of an exemplary embodiment of the initialestablishment of a communication channel between a base station and asubscriber unit using slow initial acquisition.

FIG. 46 is a flow diagram of an exemplary embodiment of thereestablishment of a communication channel between a base station and asubscriber unit using fast re-acquisition.

FIG. 47 is a diagram of the communications between a base station and aplurality of subscriber units.

FIG. 48 is a diagram of the base station and a subscriber unit which hasbeen virtually located.

FIG. 49 is a schematic overview of a plurality of subscriber units whichhave been virtually located.

FIG. 50 is a subscriber unit made in accordance with one embodiment ofthe present invention.

FIG. 51 is a flow diagram of an alternative embodiment of the initialestablishment of a communication channel between a base station and asubscriber unit using slow initial acquisition.

FIG. 52 is a flow diagram of an alternative embodiment of thereestablishment of a communication channel between a base station and asubscriber unit using fast re-acquisition.

FIG. 53 is a flow diagram of an alternative embodiment of the initialestablishment of a communication channel between a base station and asubscriber unit using slow initial acquisition.

FIG. 54 is a block diagram of a prior art data communication bus.

FIG. 55 is a table of prior art data bus architectures.

FIG. 56 is a simplified block diagram of an embodiment of the presentinvention.

FIGS. 57A-E is an electrical schematic of an embodiment of the presentinvention.

FIG. 58 is a block diagram of the message transmit DMA.

FIG. 59 is a block diagram of the message receive DMA.

FIG. 60 is a block diagram of the digital processor system.

FIG. 61 is a block diagram of a communication system in accordance withthe present invention connected to originating and terminating nodes.

FIG. 62 is a flow diagram of the establishment of a communicationchannel between originating and terminating nodes in accordance with theprior art.

FIG. 63 is a flow diagram of the establishment of a communicationchannel between originating and terminating nodes in accordance with thepresent invention.

FIG. 64 is a block diagram of a base station in accordance with theteachings of the present invention.

FIG. 65 is a block diagram of a prior art single input FIR filter.

FIG. 66 is a block diagram of a prior art single input FIR filterstructure.

FIG. 67 is a block diagram of an alternative implementation of a priorart, single input FIR filter structure.

FIG. 68A is a block diagram of a single channel of a multichannel FIRfilter.

FIG. 68B is a detailed block diagram of a multichannel FIR filter.

FIG. 69 is a block diagram showing a first refinement.

FIG. 70 is a block diagram showing a second refinement.

FIG. 71 is a block diagram of the multichannel processing element.

FIG. 72A is a global block diagram of a LUT table.

FIG. 72B is a detailed block diagram showing the multichannel LUT inputof the present invention.

FIG. 73 is a detailed block diagram of an embodiment of the presentinvention.

GLOSSARY OF ACRONYMS Acronym Definition AC Assigned Channels A/DAnalog-to-Digital ADPCM Adaptive Differential Pulse Code Modulation AFPCAutomatic Forward Power Control AGC Automatic Gain Control AMF AdaptiveMatched Filter APC Automatic Power Control ARPC Automatic Reverse PowerControl ASPT Assigned Pilot AVC Adaptive Vector Correlator AXCH AccessChannel B-CDMA Broadband Code Division Multiple Access BCM BearerChannel Modification BER Bit Error Rate BS Base Station CC Call ControlCDM Code Division Multiplex CDMA Code Division Multiple Access CLK ClockSignal Generator CO Central Office CTCH Control Channel CUCH Check-UpChannel dB Decibels DCC Data Combiner Circuitry DI DistributionInterface DLL Delay Locked Loop DM Delta Modulator DS Direct SequenceEPIC Extended PCM Interface Controller FBCH Fast Broadcast Channel FDMFrequency Division Multiplex FD/TDMA Frequency & Time Division SystemsFDMA Frequency Division Multiple Access FEC Forward Error Correction FSKFrequency Shift Keying FSU Fixed Subscriber Unit GC Global Channel GLPTGlobal Pilot GPC Global Pilot Code GPSK Gaussian Phase Shift Keying GPSGlobal Positioning System HPPC High Power Passive Components HSB HighSpeed Bus I In-Phase IC Interface Controller ISDN Integrated ServicesDigital Network ISST Initial System Signal Threshold LAXPT Long AccessPilot LAPD Link Access Protocol LCT Local Craft Terminal LE LocalExchange LFSR Linear Feedback Shift Register LI Line Interface LMS LeastMean Square LOL Loss of Code Lock LPF Low Pass Filter LSR Linear ShiftRegister MISR Modem Input Signal Receiver MIU Modem Interface Unit MMMobility Management MOI Modem Output Interface MPC Maintenance PowerControl MPSK M-ary Phase Shift Keying MSK Minimum Shift Keying MSUMobile Subscriber Unit NE Network Element OMS Operation and MaintenanceSystem OS Operations System OQPSK Offset Quadrature Phase Shift KeyingOW Order Wire PARK Portable Access Rights Key PBX Private BranchExchange PCM Pulse Coded Modulation PCS Personal Communication ServicesPG Pilot Generator PLL Phase Locked Loop PLT Pilot PN Pseudonoise POTSPlain Old Telephone Service STN Public Switched Telephone Network QQuadrature QPSK Quadrature Phase Shift Keying RAM Random Access MemoryRCS Radio Carrier Station RDI Receiver Data Input Circuit RDU RadioDistribution Unit RF Radio Frequency RLL Radio Local Loop SAXPT ShortAccess Channel Pilots SBCH Slow Broadcast Channel SHF Super HighFrequency SIR Signal Power to Interface Noise Power Ratio SLICSubscriber Line Interface Circuit SNR Signal-to-Noise Ratio SPC ServicePC SPRT Sequential Probability Ratio Test STCH Status Channel SUSubscriber Unit TDM Time Division Multiplexing TMN TelecommunicationManagement Network TRCH Traffic Channels TSI Time-Slot Interchanger TXTransmit TXIDAT I-Modem Transmit Data Signal TXQDAT Q-Modem TransmitData Signal UHF Ultra High Frequency VCO Voltage Controlled OscillatorVDC Video Distribution Circuit VGA Variable Gain Amplifier VHF Very HighFrequency WAC Wireless Access Controller

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) I. General SystemDescription

The system of the present invention provides local-loop telephoneservice using radio links between one or more base stations and multipleremote subscriber units. In the exemplary embodiment, a radio link isdescribed for a base station communicating with a fixed subscriber unit(FSU), but the system is equally applicable to systems includingmultiple base stations with radio links to both FSUs and mobilesubscriber units (MSUs). Consequently, the remote subscriber units arereferred to herein as subscriber units (SUs).

Referring to FIG. 1, base station (BS) 101 provides call connection to alocal exchange (LE) 103 or any other telephone network switchinginterface, such as a private branch exchange (PBX) and includes a radiocarrier station (RCS) 104. One or more RCSs 104, 105, 110 connect to aradio distribution unit (RDU) 102 through links 131, 132, 137, 138, 139,and RDU 102 interfaces with LE 103 by transmitting and receiving callset-up, control, and information signals through telco links 141, 142,150. SUs 116, 119 communicate with the RCS 104 through radio links 161,162, 163, 164, 165. Alternatively, another embodiment of the inventionincludes several SUs and a “master” SU with functionality similar to theRCS 104. Such an embodiment may or may not have connection to a localtelephone network.

The radio links 161 to 165 operate within the frequency bands of theDCS1800 standard (1.71-1.785 GHz and 1.805-1.880 GHz); the US-PCSstandard (1.85-1.99 GHz); and the CEPT standard (2.0-2.7 GHz). Althoughthese bands are used in the described embodiment, the invention isequally applicable to the entire UHF to SHF bands, including bands from2.7 GHz to 5 GHz. The transmit and receive bandwidths are multiples of3.5 MHz starting at 7 MHz, and multiples of 5 MHz starting at 10 MHz,respectively. The described system includes bandwidths of 7, 10, 10.5,14 and 15 MHz. In the exemplary embodiment of the invention, the minimumguard band between the uplink and downlink is 20 MHz, and is desirablyat least three times the signal bandwidth. The duplex separation isbetween 50 to 175 MHz, with the described invention using 50, 75, 80,95, and 175 MHz. Other frequencies may also be used.

Although the described embodiment uses different spread-spectrumbandwidths centered around a carrier for the transmit and receivespread-spectrum channels, the present method is readily extended tosystems using multiple spread-spectrum bandwidths for the transmitchannels and multiple spread-spectrum bandwidths for the receivechannels. Alternatively, because spread-spectrum communication systemshave the inherent feature that one user's transmission appears as noiseto another user's despreading receiver, an embodiment may employ thesame spread-spectrum channel for both the transmit and receive pathchannels. In other words, uplink and downlink transmissions can occupythe same frequency band. Furthermore, the present method may be readilyextended to multiple CDMA frequency bands, each conveying a respectivelydifferent set of messages, uplink, downlink or uplink and downlink.

The spread binary symbol information is transmitted over the radio links161 to 165 using quadrature phase shift keying (QPSK) modulation withNyquist Pulse Shaping in the present embodiment, although othermodulation techniques may be used, including, but not limited to, offsetQPSK (OQPSK), minimum shift keying (MSK). Gaussian phase shift keying(GPSK) and M-ary phase shift keying (MPSK)

The radio links 161 to 165 incorporate Broadband Code Division MultipleAccess (B-CDMA™) as the mode of transmission in both the uplink anddownlink directions. CDMA (also known as spread spectrum) communicationtechniques used in multiple access systems are well-known, and aredescribed in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUSSPREAD-SPECTRUM COMMUNICATION SYSTEM AND METHOD by Donald T Schilling.The system described utilizes the direct sequence (DS) spreadingtechnique. The CDMA modulator performs the spread-spectrum spreadingcode sequence generation, which can be a pseudonoise (PN) sequence; andcomplex DS modulation of the QPSK signals with spreading code sequencesfor the in-phase (I) and quadrature (Q) channels. Pilot signals aregenerated and transmitted with the modulated signals, and pilot signalsof the present embodiment are spreading codes not modulated by data. Thepilot signals are used for synchronization, carrier phase recovery andfor estimating the impulse response of the radio channel. Each SUincludes a single pilot generator and at least one CDMA modulator anddemodulator, together known as a CDMA modem. Each RCS 104, 105, 110 hasa single pilot generator plus sufficient CDMA modulators anddemodulators for all of the logical channels in use by all SUs.

The CDMA demodulator despreads the signal with appropriate processing tocombat or exploit multipath propagation effects. Parameters concerningthe received power level are used to generate the APC information which,in turn, is transmitted to the other end of the communication link. TheAPC information is used to control transmit power of the AFPC and ARPClinks. In addition, each RCS 104, 105 and 110 can perform maintenancepower control (MPC), in a manner similar to APC, to adjust the initialtransmit power of each SU 111, 112, 115, 117 and 118. Demodulation iscoherent where the pilot signal provides the phase reference.

The described radio links support multiple traffic channels with datarates of 8, 16, 32, 64, 128, and 144 kbs. The physical channel to whicha traffic channel is connected operates with a 64 k symbol/sec rate.Other data rates may be supported, and forward error correction (FEC)coding can be employed. For the described embodiment, FEC with codingrate of ½ and constraint length 7 is used. Other rates and constraintlengths can be used consistent with the code generation techniquesemployed.

Diversity combining at the radio antennas of RCS 104, 105 and 110 is notnecessary because CDMA has inherent frequency diversity due to thespread bandwidth. Receivers include adaptive matched filters (AMFs) (notshown in FIG. 1) which combine the multipath signals. In the presentembodiment, the exemplary AMFs perform maximal ratio combining.

Referring to FIG. 1, RCS 104 interfaces to RDU 102 through links 131,132, 137, 139, which may use, for example, 1.544 Mb/s DS1, 2.048 Mb/sE1; or HDSL formats to receive and send digital data signals. Whilethese are typical telephone company standardized interfaces, the presentinvention is not limited to these digital data formats only. Theexemplary RCS line interface (not shown in FIG. 1) translates the linecoding (such as HDB3, B8ZS, AMI) and extracts or produces framinginformation, performs alarms and facility signaling functions, as wellas channel specific loop-back and parity check functions. The interfacesfor this description provide 64 kbs PCM encoded or 32 kbs ADPCM encodedtelephone traffic channels or ISDN channels to the RCS for processing.Other ADPCM encoding techniques can be used consistent with the sequencegeneration techniques.

The system of the present invention also supports bearer ratemodification between the RCS 104 and each SU 111, 112, 115, 117 and 118communicating with the RCS 104 in which a CDMA message channelsupporting 64 kbs may be assigned to voiceband data or FAX when ratesabove 4.8 kbs are present. Such 64 kbs bearer channel is considered anunencoded channel. For ISDN, bearer rate modification may be donedynamically, based upon the D channel messages.

In FIG. 1, each SU 111, 112, 115, 117 and 118 either includes orinterfaces with a telephone unit 170, or interfaces with a local switch(PBX) 171. The input from the telephone unit may include voice,voiceband data and signaling. The SU translates the analog signals intodigital sequences, and may also include a data terminal 172 or an ISDNinterface 173. The SU can differentiate voice input, voiceband data orFAX and digital data. The SU encodes voice data with techniques such asADPCM at 32 kbs or lower rates, and detects voiceband data or FAX withrates above 4.8 kbs to modify the traffic channel (bearer ratemodification) for unencoded transmission. Also, A-law, u-law or nocompounding of the signal may be performed before transmission. Fordigital data, data compression techniques, such as idle flag removal,may also be used to conserve capacity and minimize interference.

The transmit power levels of the radio interface between RCS 104 and SUs111, 112, 115, 117 and 118 are controlled using two different closedloop power control methods. The AFPC method determines the downlinktransmit power level, and the ARPC method determines the uplink transmitpower level. The logical control channel by which SU 111 and RCS 104,for example, transfer power control information operates at least a 16kHz update rate. Other embodiments may use a faster or slower updaterate, for example 64 kHz. These algorithms ensure that the transmitpower of a user maintains an acceptable bit-error rate (BER), maintainsthe system power at a minimum to conserve power and maintains the powerlevel of all SUs 111, 112, 115, 117 and 118 received by RCS 104 at anearly equal level.

In addition, the system uses an optional maintenance power controlmethod during the inactive mode of a SU. When SU 111 is inactive orpowered-down to conserve power, the unit occasionally activates toadjust its initial transmit power level setting in response to amaintenance power control signal from RCS 104. The maintenance powersignal is determined by the RCS 104 by measuring the received powerlevel of SU 111 and present system power level and, from this,calculates the necessary initial transmit power. The method shortens thechannel acquisition time of SU 111 to begin a communication. The methodalso prevents the transmit power level of SU 111 from becoming too highand interfering with other channels during the initial transmissionbefore the closed loop power control reduces the transmit power.

RCS 104 obtains synchronization of its clock from an interface line suchas, but not limited to, E1, T1, or HDSL interfaces. RCS 104 can alsogenerate its own internal clock signal from an oscillator which may beregulated by a global positioning system (GPS) receiver. RCS 104generates a global pilot code, a channel with a spreading code but nodata modulation, which can be acquired by remote SUs 111 through 118.All transmission channels of the RCS are synchronized to the pilotchannel, and spreading code phases of code generators (not shown) usedfor logical communication channels within RCS 104 are also synchronizedto the pilot channel's spreading code phase. Similarly, SUs 111 through118 which receive the global pilot code of RCS 104 synchronize thespreading and de-spreading code phases of the code generators (notshown) of the SUs to the global pilot code.

RCS 104, SU 111 and RDU 102 may incorporate system redundancy of systemelements and automatic switching between internal functional systemelements upon a failure event to prevent loss or drop-out of a radiolink, power supply, traffic channel or group of traffic channels.

II. Logical Communication Channels

A ‘channel’ of the prior art is usually regarded as a communicationspath which is part of an interface and which can be distinguished fromother paths of that interface without regard to its content. However, inthe case of CDMA, separate communications paths are distinguished onlyby their content. The term ‘logical channel’ is used to distinguish theseparate data streams, which are logically equivalent to channels in theconventional sense. All logical channels and sub-channels of the presentinvention are mapped to a common 64 kilo-symbols per second (ksym/s)QPSK stream. Some channels are synchronized to associated pilot codeswhich are generated from, and perform a similar function to the systemglobal pilot code (GPC). The system pilot signals are not, however,considered logical channels.

Several logical communication channels are used over the RFcommunication link between the RCS and SU. Each logical communicationchannel either has a fixed, pre-determined spreading code or adynamically assigned spreading code. For both pre-determined andassigned codes, the code phase is synchronized with the pilot code.Logical communication channels are divided into two groups: the globalchannel (GC) group includes channels which are either transmitted fromthe base station RCS to all remote SUs or from any SU to the RCS of thebase station regardless of the SU's identity. The channels in the GCgroup may contain information of a given type for all users includingthose channels used by SUs to gain system access. Channels in theassigned channels (AC) group are those channels dedicated tocommunication between the RCS and a particular SU.

The global channels (GC) group provides for 1) broadcast control logicalchannels, which provide point-to-multipoint services for broadcastingmessages to all SUs and paging messages to SUs; and 2) access controllogical channels which provide point-to-point services on globalchannels for SUs to access the system and obtain assigned channels. TheRCS of the present invention has multiple access control logicalchannels, and one broadcast control group. An SU of the presentinvention has at least one access control channel and at least onebroadcast control logical channel.

The global logical channels controlled by the RCS are the fast broadcastchannel (FBCH) which broadcasts fast changing information concerningwhich services and which access channels are currently available, andthe slow broadcast channel (SBCH) which broadcasts slow changing systeminformation and paging messages. The access channel (AXCH) is used bythe SUs to access an RCS and gain access to assigned channels. Each AXCHis paired with a control channel (CTCH). The CTCH is used by the RCS toacknowledge and reply to access attempts by SUs. The long access pilot(LAXPT) is transmitted synchronously with AXCH to provide the RCS with atime and phase reference. An assigned channel (AC) group contains thelogical channels that control a single telecommunication connectionbetween the RCS and a SU. The functions developed when an AC group isformed include a pair of power control logical message channels for eachof the uplink and downlink connections, and depending on the type ofconnection, one or more pairs of traffic channels. The bearer controlfunction performs the required forward error control, bearer ratemodification, and encryption functions.

Each SU 111, 112, 115, 117 and 118 has at least one AC group formed whena telecommunication connection exists, and each RCS 104, 105 and 110 hasmultiple AC groups formed, one for each connection in progress. An ACgroup of logical channels is created for a connection upon successfulestablishment of the connection. The AC group includes encryption, FECcoding and multiplexing on transmission, and FEC decoding, decryptionand demultiplexing on reception.

Each AC group provides a set of connection oriented point-to-pointservices and operates in both directions between a specific RCS, forexample, RCS 104 and a specific SU, for example, SU 111. An AC groupformed for a connection can control more than one bearer over the RFcommunication channel associated with a single connection. Multiplebearers are used to carry distributed data such as, but not limited to,ISDN. An AC group can provide for the duplication of traffic channels tofacilitate switch over to 64 kbs PCM for high speed facsimile and modemservices for the bearer rate modification function.

The assigned logical channels formed upon a successful call connectionand included in the AC group are a dedicated signaling channel [orderwire (OW)], an APC channel, and one or more traffic channels (TRCH)which are bearers of 8, 16, 32, or 64 kbs depending on the servicesupported. For voice traffic, moderate rate coded speech, ADPCM or PCMcan be supported on the traffic channels. For ISDN service types, two 64kbs TRCHs form the B channels and a 16 kbs TRCH forms the D channel.Alternatively, the APC subchannel may either be separately modulated onits own CDMA channel, or may be time division multiplexed with a trafficchannel or OW channel.

Each SU 111, 112, 115, 117 and 118 of the present invention supports upto three simultaneous traffic channels. The mapping of the three logicalchannels for TRCHs to the user data is shown below in Table 1:

TABLE 1 Mapping of service types to the three available TRCH channelsService TRCH(0) TRCH(1) TRCH(2) 16 kbs POTS TRCH/16 not used not used32 + 64 kbs POTS (during TRCH/32 TRCH/64 not used BCM) 32 kbs POTSTRCH/32 not used not used 64 kbs POTS not used TRCH/64 not used ISDN Dnot used not used TRCH/16 ISDN B + D TRCH/64 not used TRCH/16 ISDN 2B +D TRCH/64 TRCH/64 TRCH/16 Digital LL @ 64 kbs TRCH/64 not used not usedDigital LL @ 2 × 64 kbs TRCH/64 TRCH/64 not used Analog LL @ 64 kbsTRCH/64 not used not used

The APC data rate is sent at 64 kbs. The APC logical channel is not FECcoded to avoid delay and is transmitted at a relatively low power levelto minimize capacity used for APC. Alternatively, the APC and OW may beseparately modulated using complex spreading code sequences or they maybe time division multiplexed.

The OW logical channel is FEC coded with a rate ½ convolutional code.This logical channel is transmitted in bursts when signaling data ispresent to reduce interference. After an idle period, the OW signalbegins with at least 35 symbols prior to the start of the data frame.For silent maintenance call data, the OW is transmitted continuouslybetween frames of data. Table 2 summarizes the logical channels used inthe exemplary embodiment:

TABLE 2 Logical Channels and sub-channels of the B-CDMA Air InterfaceDirection (forward Channel Brief or Bit Max name Abbr. Descriptionreverse) rate BER Power level Pilot Global Channels Fast FBCH BroadcastsF 16 1e-4 Fixed GLPT Broadcast fast-changing kbs Channel systeminformation Slow SBCH Broadcasts F 16 1e-7 Fixed GLPT Broadcast pagingkbs Channel messages to FSUs and slow-changing system information AccessAXCH(i) For initial R 32 1e-7 Controlled LAXPT(i) Channels access kbs byAPC attempts by FSUs Control CTCH(i) For granting F 32 1e-7 Fixed GLPTChannels access kbs Assigned Channels 16 kbs TRCH/ General POTS F/R 161e-4 Controlled F-GLPT POTS 16 use kbs by APC R-ASPT 32 kbs TRCH/General POTS F/R 32 1e-4 Controlled F-GLPT POTS 32 use kbs by APC R-ASPT64 kbs TRCH/ POTS use for F/R 64 1e-4 Controlled F-GLPT POTS 64 in-bandkbs by APC R-ASPT modems/fax Channel Abbr. Brief Direction Bit Max Powerlevel Pilot name Description (forward or rate BER reverse) D channelTRCH ISDN D F/R 16 1e-7 Controlled F-GLPT /16 channel kbs by APC R-ASPTOrder OW assigned F/R 32 1e-7 Controlled F-GLPT wire signaling kbs byAPC R-ASPT channel channel APC APC carries APC F/R 64 2e-1 ControlledF-GLPT channel commands kbs by APC R-ASPT

III. The Spreading Codes

The CDMA code generators used to encode the logical channels of thepresent invention employ linear shift registers (LSRs) with feedbacklogic which is a method well known in the art. The code generators ofthe present embodiment of the invention generate 64 synchronous uniquesequences. Each RF communication channel uses a pair of these sequencesfor complex spreading (in-phase and quadrature) of the logical channels,so the generator gives 32 complex spreading sequences. The sequences aregenerated by a single seed which is initially loaded into a shiftregister circuit.

IV. The Generation of Spreading Code Sequences and Seed Selection

The spreading code period of the present invention is defined as aninteger multiple of the symbol duration, and the beginning of the codeperiod is also the beginning of the symbol. The relation betweenbandwidths and the symbol lengths chosen for the exemplary embodiment ofthe present invention is:

BW (MHZ) L (chips/symbol) 7  91 10 130 10.5 133 14 182 15 195

The spreading code length is also a multiple of 64 and of 96 for ISDNframe support. The spreading code is a sequence of symbols, called chipsor chip values. The general methods of generating pseudorandom sequencesusing Galois Field mathematics is known to those skilled in the art.First, the length of the LFSR to generate a code sequence is chosen, andthe initial value of the register is called a “seed”. Second, theconstraint is imposed that no code sequence generated by a code seed maybe a cyclic shift of another code sequence generated by the same codeseed. Finally, no code sequence generated from one seed may be a cyclicshift of a code sequence generated by another seed. It has beendetermined that the spreading code length of chip values of the presentinvention is:128×233,415=29,877,120  Equation (1)

The FBCH channel of the exemplary embodiment is an exception because itis not coded with the 128 length sequence, so the FBCH channel spreadingcode has period 233415.

The nonlinear sequence of length 128 is implemented as a fixed sequenceloaded into a shift register with a feed-back connection. The fixedsequence can be generated by an m-sequence of length 127 padded with anextra logic 0, 1, or random value as is well known in the art.

The linear sequence of length L=233415 is generated using an LFSRcircuit with 36 stages. The feedback connections correspond to anirreducible polynomial h(n) of degree 36. The polynomial h(x) chosen forthe exemplary embodiment of the present invention is

h(x) = x³⁶ + x³⁵ + x³⁰ + x²⁸ + x²⁶ + x²⁵ + x²² + x²⁰ + x¹⁹ + x¹⁷ + x¹⁶ + x¹⁵ + x¹⁴ + x¹² + x¹¹ + x⁹ + x⁸ + x⁴ + x³ + x² + 1or, in binary notationh(x)=(1100001010110010110111101101100011101)  Equation (2)

A group of “seed” values for a LFSR representing the polynomial h(x) ofEquation (2) which generates code sequences that are nearly orthogonalwith each other is determined. The first requirement of the seed valuesis that the seed values do not generate two code sequences which aresimply cyclic shifts of each other.

The seeds are represented as elements of GF(2³⁶) which is the field ofresidue classes modulo h(x). This field has a primitive element δ=x²+x+1or, in binary notationδ=(000000000000000000000000000000000111)  Equation (3)

Every element of GF(2³⁶) can also be written as a power of δ reducedmodulo h(x). Consequently, the seeds are represented as powers of δ, theprimitive element.

The solution for the order of an element does not require a search ofall values; the order of an element divides the order of the field(GF(2³⁶)). When δ is any element of GF(2³⁶) withχ^(e)≡1  Equation (4)for some e, then e|2³⁶−1. Therefore, the order of any element in GF(2³⁶)divides 2³⁶−1. Using these constraints, it has been determined that anumerical search generates a group of seed values, n, which are powersof δ, the primitive element of h(x).

The present invention includes a method to increase the number ofavailable seeds for use in a CDMA communication system by recognizingthat certain cyclic shifts of the previously determined code sequencesmay be used simultaneously. The round trip delay for the cell sizes andbandwidths of the present invention are less than 3000 chips. In oneembodiment of the present invention, sufficiently separated cyclicshifts of a sequence can be used within the same cell without causingambiguity for a receiver attempting to determine the code sequence. Thismethod enlarges the set of sequences available for use. By implementingthe tests previously described, a total of 3879 primary seeds weredetermined through numerical computation. These seeds are givenmathematically as:δ^(n) modulo h(x)  Equation (5)where 3879 values of n, with δ=(00, . . . 00111) as in (3), is a seriesincrementing by 1, starting at 1 and continuing to 1101, resuming at2204 and continuing to 3305, resuming at 4408 and continuing to 5509,and resuming at 6612 and ending at 7184.

When all primary seeds are known, all secondary seeds of the presentinvention are derived from the primary seeds by shifting them multiplesof 4095 chips modulo h(x). Once a family of seed values is determined,these values are stored in memory and assigned to logical channels asnecessary. Once assigned, the initial seed value is simply loaded intoLFSR to produce the required spreading code associated with the seedvalue.

V. Rapid Acquisition Feature of Long and Short Codes

Rapid acquisition of the correct code phase by a spread-spectrumreceiver is improved by designing spreading codes which are faster todetect. It should be noted that spreading code, code sequence, spreadingcode sequence, chip code, chip sequence or chip code sequence may beused interchangeably to refer to a modulation signal used to modulate aninformation signal, whereby the period of the modulation signal issubstantially less than the period of the information signal. Forsimplicity, the term spreading code will be used. The present embodimentof the invention includes a new method of generating spreading codesthat have rapid acquisition properties by using one or more of thefollowing methods. First, a long code may be constructed from two ormore short codes. The new implementation uses many spreading codes, oneor more of which are rapid acquisition sequences of length L that haveaverage acquisition phase searches r=log 2 L. Sequences with suchproperties are well known to those practiced in the art. The averagenumber of acquisition test phases of the resulting long sequence is amultiple of r=log 2 L rather than half of the number of phases of thelong sequence.

Second, a method of transmitting complex valued spreading codes(in-phase (I) and quadrature (Q) sequences) in a pilot spreading codesignal may be used rather than transmitting real valued sequences. Twoor more separate spreading codes may be transmitted over the complexchannels. If the codes have different phases, an acquisition may be doneby acquisition circuits in parallel over the different spreading codeswhen the relative phase shift between the two or more code channels isknown. For example, for two spreading codes, one can be sent on an inphase (I) channel and one on the quadrature (Q) channel. To search thespreading codes, the acquisition detection means searches the twochannels, but begins the Q channel with an offset equal to one-half ofthe spreading code length. With code length of N, the acquisition meansstarts the search at N/2 on the Q channel. The average number of teststo find acquisition is N/2 for a single code search, but searching the Iand phase delayed Q channel in parallel reduces the average number oftests to N/4. The codes sent on each channel could be the same code, thesame code with one channel's code phase delayed or different spreadingcodes.

VI. Epoch and Sub-Epoch Structures

The long complex spreading codes used for the exemplary system of thepresent invention have a number of chips after which the code repeats.The repetition period of the spreading code is called an epoch. To mapthe logical channels to CDMA spreading codes, the present invention usesan epoch and sub-epoch structure. The code period for the CDMA spreadingcode to modulate logical channels is 29877120 chips/code period, whichis the same number of chips for all bandwidths. The code period is theepoch of the present invention, and Table 3 below defines the epochduration for the supported chip rates. In addition, two sub-epochs aredefined over the spreading code epoch and are 233415 chips and 128 chipslong.

The 233415 chip sub-epoch is referred to as a long sub-epoch, and isused for synchronizing events on the RF communication interface such asencryption key switching and changing from global to assigned codes. The128 chip short epoch is defined for use as an additional timingreference. The highest symbol rate used with a single CDMA code is 64ksym/s. There are always an integer number of chips in a symbol durationfor the supported symbol rates 64, 32, 16, and 8 ksym/s.

TABLE 3 Bandwidths, Chip Rates, and Epochs number of 128 chip 233415chip Chip Rate, chips in a sub-epoch sub-epoch Epoch Bandwidth Complex64 kbit/sec duration* duration* duration (MHz) (Mchip/sec) symbol (ms)(ms) (sec) 7 5.824 91 21.978 40.078 5.130 10 8.320 130 15.385 28.0553.591 10.5 8.512 133 15.038 27.422 3.510 14 11.648 182 10.989 20.0392.565 15 12.480 195 10.256 18.703 2.394 *numbers in these columns arerounded to 5 digits.

VII. Mapping of the Logical Channels to Epochs and Sub-Epochs

The complex spreading codes are designed such that the beginning of thecode epoch coincides with the beginning of a symbol for all of thebandwidths supported. The present invention supports bandwidths of 7,10, 10.5, 14, and 15 MHz. Assuming nominal 20% roll-off, thesebandwidths correspond to the following chip rates in Table 4.

TABLE 4 Supported Bandwidths and Chip Rates for CDMA. R_(c) (ComplexExcess BW, Factorization BW (MHz) Mchips/sec) % L:(R_(c)/L) = 64k of L 75.824 20.19  91 7 × 13 10 8.320 20.19 130 2 × 5 × 13 10.5 8.512 23.36133 7 × 19 14 11.648  20.19 182 2 × 7 × 13 15 12.480  20.19 195 3 × 5 ×13The number of chips in an epoch is:N=29877120=2⁷×3³×5×7×13×19  Equation (6)

If interleaving is used, the beginning of an interleaver periodcoincides with the beginning of the sequence epoch. The spreadingsequences generated using the method of the present invention cansupport interleaver periods that are multiples of 1.5 ms for variousbandwidths.

Cyclic sequences of the prior art are generated using LFSR circuits.However, this method does not generate sequences of even length. Oneembodiment of the spreading code generator using the code seedsgenerated previously is shown in FIG. 2 a, FIG. 2 b, and FIG. 2 c. Thepresent invention uses a 36 stage LFSR 201 to generate a sequence ofperiod N′=233415.3³×5×7×13×19, which is C_(o) in FIG. 2 a. In FIGS. 2 a,2 b, and 2 c, the symbol ⊕ represents a binary addition (EXCLUSIVE-OR).A spreading code generator designed as above generates the in-phase andquadrature parts of a set of complex sequences. The tap connections andinitial state of the 36 stage LFSR determine the sequence generated bythis circuit. The tap coefficients of the 36 stage LFSR are determinedsuch that the resulting sequences have the period 233415. Note that thetap connections shown in FIG. 2 a correspond to the polynomial given inEquation (2). Each resulting sequence is then overlaid by binaryaddition with the 128 length sequence C* to obtain the epoch period29877120.

FIG. 2 b shows a feed forward (FF) circuit 202 which is used in the codegenerator. The signal X[n−1] is output of the chip delay 211, and theinput of the chip delay 211 is X[n]. The code chip C[n] is formed by thelogical adder 212 from the input X[n] and X[n−1]. FIG. 2 c shows thecomplete spreading code generator. From the LFSR 201, output signals gothrough a chain of up to 63 single stage FFs 203 cascaded as shown. Theoutput of each FF is overlaid with the short, even code sequence C*period 128=2⁷ which is stored in code memory 222 and which exhibitsspectral characteristics of a pseudorandom sequence to obtain the epochN=29877120. This sequence of 128 is determined by using an m-sequence(PN sequence) of length 127=2⁷−1 and adding a bit-value, such as logic0, to the sequence to increase the length to 128 chips. The even codesequence C* is input to the even code shift register 221, which is acyclic register, that continually outputs the sequence. The shortsequence is then combined with the long sequence using an EXCLUSIVE-ORoperation 213, 214, 220.

As shown in FIG. 2 c, up to 63 spreading codes C_(o) through C₆₃ aregenerated by tapping the output signals of FFs 203 and logically addingthe short sequence C* in binary adders 213, 214, and 220, for example.One skilled in the art would realize that the implementation of FF 203will create a cumulative delay effect for the spreading codes producedat each FF stage in the chain. This delay is due to the nonzeroelectrical delay in the electronic components of the implementation. Thetiming problems associated with the delay can be mitigated by insertingadditional delay elements into the FF chain in one version of theembodiment of the invention. The FF chain of FIG. 2 c with additionaldelay elements is shown in FIG. 2 d.

The code-generators in the exemplary embodiment of the present inventionare configured to generate either global codes or assigned codes. Globalcodes are CDMA codes that can be received or transmitted by all users ofthe system. Assigned codes are CDMA codes that are allocated for aparticular connection. When a set of spreading codes are generated fromthe same generator as described, only the seed of the 36 stage LFSR isspecified to generate a family of spreading codes. Spreading codes forall of the global codes are generated using the same LFSR circuit.Therefore, once an SU has synchronized to the global pilot signal froman RCS and knows the seed for the LFSR circuit for the global channelcodes, it can generate not only the pilot spreading code but also allother global codes used by the RCS.

The signal that is upconverted to RF is generated as follows. The outputsignals of the above shift register circuits are converted to anantipodal sequence (0 maps into +1, 1 maps into −1). The logicalchannels are initially converted to QPSK signals, which are mapped asconstellation points as is well known in the art. The in-phase andquadrature channels of each QPSK signal form the real and imaginaryparts of the complex data value. Similarly, two spreading codes are usedto form complex spreading chip values. The complex data are spread bybeing multiplied by the complex spreading code. Similarly, the receivedcomplex data is correlated with the conjugate of the complex spreadingcode to recover despread data.

VIII. Short Codes

Short codes are used for the initial ramp-up process when a SU accessesan RCS. The period of the short codes is equal to the symbol durationand the start of each period is aligned with a symbol boundary. Both SUand RCS derive the real and imaginary parts of the short codes from thelast eight feed-forward sections of the code generator producing theglobal codes for that cell.

The short codes that are in use in the exemplary embodiment of theinvention are updated every 3 ms. Other update times that are consistentwith the symbol rate may be used. Therefore, a change-over occurs every3 ms starting from the epoch boundary. At a change-over, the next symbollength portion of the corresponding feed-forward output becomes theshort code. When the SU needs to use a particular short code, it waitsuntil the first 3 ms boundary of the next epoch and stores the nextsymbol length portion output from the corresponding FF section. Thisshall be used as the short code until the next change-over, which occurs3 ms later.

The signals represented by these short codes are known as short accesschannel pilots (SAXPTs).

IX. Mapping of Logical Channels to Spreading Codes

The exact relationship between the spreading codes and the CDMA logicalchannels and pilot signals is documented in Table 5a and Table 5b. Thosesignal names ending in ‘-CH’ correspond to logical channels. Thosesignal names ending in ‘-PT’ correspond to pilot signals, which aredescribed in detail below.

TABLE 5a Spreading code sequences and global CDMA codes Logical ChannelSequence Quadrature or Pilot Signal Direction C₀ I FBCH Forward (F) C₁ QFBCH F C₂⊕C* I GLPT F C₃⊕C* Q GLPT F C₄⊕C* I SBCH F C₅⊕C* Q SBCH F C₆⊕C*I CTCH (0) F C₇⊕C* Q CTCH (0) F C₈⊕C* I APCH (1) F C₉⊕C* Q APCH (1) FC₁₀⊕C* I CTCH (1) F C₁₁⊕C* Q CTCH (1) F C₁₂⊕C* I APCH (1) F C₁₃⊕C* QAPCH (1) F C₁₄⊕C* I CTCH (2) F C₁₅⊕C* Q CTCH (2) F C₁₆⊕C* I APCH (2) FC₁₇⊕C* Q APCH (2) F C₁₈⊕C* I CTCH (3) F C₁₉⊕C* Q CTCH (3) F C₂₀⊕C* IAPCH (3) F C₂₁⊕C* Q APCH (3) F C₂₂⊕C* I reserved — C₂₃⊕C* Q reserved — .. . . . . . . . . . . . . . . . . . . . . . . C₄₀⊕C* I reserved — C₄₁⊕C*Q reserved — C₄₂⊕C* I AXCH(3) Reverse (R) C₄₃⊕C* Q AXCH(3) R C₄₄⊕C* ILAXPT(3) R SAXPT(3) seed C₄₅⊕C* Q LAXPT(3) R SAXPT(3) seed C₄₆⊕C* IAXCH(2) R C₄₇⊕C* Q AXCH(2) R C₄₈⊕C* I LAXPT(2) R SAXPT(2) seed C₄₉⊕C* QLAXPT(2) R SAXPT(2) seed C₅₀⊕C* I AXCH(1) R C₅₁⊕C* Q AXCH(1) R C₅₂⊕C* ILAXPT(1) R SAXPT(1) seed C₅₃⊕C* Q LAXPT(1) R SAXPT(1) seed C₅₄⊕C* IAXCH(0) R C₅₅⊕C* Q AXCH(0) R C₅₆⊕C* I LAXPT(0) R SAXPT(0) seed C₅₇⊕C* QLAXPT(0) R SAXPT(0) seed C₅₈⊕C* I IDLE — C₅₉⊕C* Q IDLE — C₆₀⊕C* I AUX RC₆₁⊕C* Q AUX R C₆₂⊕C* I reserved — C₆₃⊕C* Q reserved —

TABLE 5b Spreading code sequences and assigned CDMA codes. LogicalChannel Sequence Quadrature or Pilot Signal Direction C₀⊕C* I ASPTReverse (R) C₁⊕C* Q ASPT R C₂⊕C* I APCH R C₃⊕C* Q APCH R C₄⊕C* I OWCH RC₅⊕C* Q OWCH R C₆⊕C* I TRCH(0) R C₇⊕C* Q TRCH(0) R C₈⊕C* I TRCH(1) RC₉⊕C* Q TRCH(1) R C₁₀⊕C* I TRCH(2) R C₁₁⊕C* Q TRCH(2) R C₁₂⊕C* I TRCH(3)R C₁₃⊕C* Q TRCH(3) R C₁₄⊕C* I reserved — C₁₅⊕C* Q reserved — . . . . . .. . . . . . . . . . . . . . . . . . C₄₄⊕C* I reserved — C₄₅⊕C* Qreserved — C₄₆⊕C* I TRCH(3) Forward (F) C₄₇⊕C* Q TRCH(3) F C₄₈⊕C* ITRCH(2) F C₄₉⊕C* Q TRCH(2) F C₅₀⊕C* I TRCH(1) F C₅₁⊕C* Q TRCH(1) FC₅₂⊕C* I TRCH(0) F C₅₃⊕C* Q TRCH(0) F C₅₄⊕C* I OWCH F C₅₅⊕C* Q OWCH FC₅₆⊕C* I APCH F C₅₇⊕C* Q APCH F C₅₈⊕C* I IDLE — C₅₉⊕C* Q IDLE — C₆₀⊕C* Ireserved — C₆₁⊕C* Q reserved — C₆₂⊕C* I reserved — C₆₃⊕C* Q reserved —

For global codes, the seed values for the 36 bit shift register arechosen to avoid using the same code, or any cyclic shift of the samecode, within the same geographical area to prevent ambiguity or harmfulinterference. No assigned code is equal to, or a cyclic shift of, aglobal code.

X. Pilot Signals

The pilot signals are used for synchronization, carrier phase recoveryand for estimating the impulse response of the radio channel. The RCS104 transmits a forward link pilot carrier reference as a complex pilotcode sequence to provide time and phase reference for all SUs 111, 112,115, 117 and 118 in its service area. The power level of the globalpilot (GLPT) signal is set to provide adequate coverage over the wholeRCS service area, which area depends on the cell size. With only onepilot signal in the forward link, the reduction in system capacity dueto the pilot energy is negligible.

The SUs 111, 112, 115, 117 and 118 each transmit a pilot carrierreference as a quadrature modulated (complex-valued) pilot spreadingcode sequence to provide a time and phase reference to the RCS for thereverse link. The pilot signal transmitted by the SU of one embodimentof the invention is 6 dB lower than the power of the 32 kbs POTS trafficchannel. The reverse pilot channel is subject to APC. The reverse linkpilot associated with a particular connection is called the assignedpilot (ASPT). In addition, there are pilot signals associated withaccess channels. These are called the long access channel pilots(LAXPTs). Short access channel pilots (SAXPTs) are also associated withthe access channels and used for spreading code acquisition and initialpower ramp-up. All pilot signals are formed from complex codes, asdefined below:GLPT(forward)={C ₂ ⊕C*)+j.(C ₃ ⊕C*)}.{(1)+j.(0)}{Complex Code}.{Carrier}

The complex pilot signals are de-spread by multiplication with conjugatespreading codes: {(C₂⊕C*)−j.(C₃⊕C*)}. By contrast, traffic channels areof the form:TRCH_(n)(forward/reverse)={(C _(k) ⊕C*)+j.(C ₁⊕C*)}.{(±1)+j(±1)}{Complex Codes}.{Data Symbol}which thus form a constellation set at

$\frac{\pi}{4}$radians with respect to the pilot signal constellations. The GLPTconstellation is shown in FIG. 3 a, and the TRCH_(n) traffic channelconstellation is shown in FIG. 3 b.

XI. Logical Channel Assignment of the FBCH, SBCH, and Traffic Channels

The FBCH is a global forward link channel used to broadcast dynamicinformation about the availability of services and AXCHs. Messages aresent continuously over this channel, and each message lastsapproximately 1 ms. The FBCH message is 16 bits long, repeatedcontinuously, and is epoch aligned. The FBCH is formatted as defined inTable 6.

TABLE 6 FBCH format Bit Definition 0 Traffic Light 0 1 Traffic Light 1 2Traffic Light 2 3 Traffic Light 3 4-7 service indicator bits 8 TrafficLight 0 9 Traffic Light 1 10  Traffic Light 2 11  Traffic Light 3 12-15service indicator bits

For the FBCH, bit 0 is transmitted first. As used in Table 6, a trafficlight corresponds to an access channel (AXCH) and indicates whether theparticular access channel is currently in use (a red) or not in use (agreen). A logic ‘1’ indicates that the traffic light is green, and alogic ‘0’ indicates the traffic light is red. The values of the trafficlight bits may change from octet to octet and each 16 bit messagecontains distinct service indicator bits which describe the types ofservices that are available for the AXCHs.

One embodiment of the present invention uses service indicator bits asfollows to indicate the availability of services or AXCHs. The serviceindicator bits {4, 5, 6, 7, 12, 13, 14, 15} taken together may be anunsigned binary number, with bit 4 as the MSB and bit 15 as the LSB.Each service type increment has an associated nominal measure of thecapacity required, and the FBCH continuously broadcasts the availablecapacity. This is scaled to have a maximum value equivalent to thelargest single service increment possible. When a SU requires a newservice or an increase in the number of bearers it compares the capacityrequired to that indicated by the FBCH and then considers itself blockedif the capacity is not available. The FBCH and the traffic channels arealigned to the epoch.

Slow broadcast information frames contain system or other generalinformation that is available to all SUs and paging information framescontain information about call requests for particular SUs. Slowbroadcast information frames and paging information frames aremultiplexed together on a single logical channel which forms the slowbroadcast channel (SBCH). As previously defined, the code epoch is asequence of 29,877,120 chips having an epoch duration which is afunction of the chip rate defined in Table 7 below. In order tofacilitate power saving, the channel is divided into N “Sleep” cycles,and each cycle is subdivided into M slots, which are 19 ms long, exceptfor 10.5 MHz bandwidth which has slots of 18 ms.

TABLE 7 SBCH Channel Format Outline Spreading Code Epoch Cycles/ CycleSlots/ Slot Bandwidth Rate Length Epoch Length Cycle Length (MHz) (MHz)(ms) N (ms) M (ms) 7.0 5.824 5130 5 1026 54 19 10.0 8.320 3591 3 1197 6319 10.5 8.512 3510 3 1170 65 18 14.0 11.648 2565 3  855 45 19 15.012.480 2394 2 1197 63 19

Sleep cycle slot #1 is always used for slow broadcast information. Slots#2 to #M-1 are used for paging groups unless extended slow broadcastinformation is inserted. The pattern of cycles and slots in oneembodiment of the present invention run continuously at 16 kbs.

Within each sleep cycle the SU powers-up the receiver and re-acquiresthe pilot code. It then achieves carrier lock to a sufficient precisionfor satisfactory demodulation and Viterbi decoding. The settling time toachieve carrier lock may be up to 3 slots in duration. For example, anSU assigned to Slot #7 powers up the receiver at the start of slot #4.Having monitored its slot the SU will have either recognized its pagingaddress and initiated an access request, or failed to recognize itspaging address in which case it reverts to the sleep mode. Table 8 showsduty cycles for the different bandwidths, assuming a wake-up duration of3 slots.

TABLE 8 Sleep-Cycle Power Saving Bandwidth (MHz) Slots/Cycle Duty Cycle 7.0 54 7.4% 10.0 63 6.3% 10.5 65 6.2% 14.0 45 8.9% 15.0 63 6.3%

XII. Spreading Code Tracking and AMF Detection in Multipath Channels

Three CDMA spreading code tracking methods in multipath fadingenvironments are described which track the code phase of a receivedmultipath spread-spectrum signal. The first is the prior art trackingcircuit which simply tracks the spreading code phase with the highestdetector output signal value, the second is a tracking circuit thattracks the median value of the code phase of the group of multipathsignals, and the third is the centroid tracking circuit which tracks thecode-phase of an optimized, least mean squared weighted average of themultipath signal components. The following describes the algorithms bywhich the spreading code phase of the received CDMA signal is tracked.

A tracking circuit has operating characteristics that reveal therelationship between the time error and the control voltage that drivesa voltage controlled oscillator (VCO) of a spreading code phase trackingcircuit. When there is a positive timing error, the tracking circuitgenerates a negative control voltage to offset the timing error. Whenthere is a negative timing error, the tracking circuit generates apositive control voltage to offset the timing error. When the trackingcircuit generates a zero value, this value corresponds to the perfecttime alignment called the ‘lock out’.

FIG. 3 c shows the basic tracking circuit. Received signal r(t) isapplied to matched filter 301, which correlates r(t) with a localcode-sequence c(t) generated by code generator 303. The output signal ofthe matched filter x(t) is sampled at the sampler 302 to produce samplesx[nT] and x[nT+T/2]. The samples x[nT] and x[nT+T/2] are used by atracking circuit 304 to determine if the phase of the spreading codec(t) of the code generator 303 is correct. The tracking circuit 304produces an error signal e(t) as an input to the code generator 303. Thecode generator 303 uses this signal e(t) as an input signal to adjustthe code-phase it generates.

In a CDMA system, the signal transmitted by the reference user iswritten in the low-pass representation as:

$\begin{matrix}{{s(t)} = {\sum\limits_{k = {- \infty}}^{\infty}{C_{k}{P_{Tc}( {t - {k\; T_{c}}} )}}}} & \text{Equation~~(7)}\end{matrix}$where C_(k) represents the spreading code coefficients, P_(Tc)(t)represents the spreading code chip waveform and T_(c) is the chipduration. Assuming that the reference user is not transmitting data sothat only the spreading code modulates the carrier. Referring to FIG. 3c, the received signal is:

$\begin{matrix}{{r(t)} = {\sum\limits_{i = 1}^{M}{a_{i}{s( {t - \tau_{i}} )}}}} & \text{Equation~~(8)}\end{matrix}$Here, a_(i) is due to fading effect of the multipath channel on the i-thpath and τ_(i) is the random time delay associated with the same path.The receiver passes the received signal through a matched filter, whichis implemented as a correlation receiver and is described below. Thisoperation is done in two steps: first the signal is passed through achip matched filter and sampled to recover the spreading code chipvalues; then this spreading code is correlated with the locallygenerated spreading code.

FIG. 3 c shows the chip matched filter 301, matched to the chip waveformP_(Tc)(t), and the sampler 302. Ideally, the signal x(t) at the outputterminal of the chip matched filter 301 is:

$\begin{matrix}{{x(t)} = {\sum\limits_{i = k}^{M}{\sum\limits_{k = {- \infty}}^{\infty}{a_{i}c_{k}{g( {t - \tau_{i} - {k\; T_{c}}} )}}}}} & \text{Equation~~(9)}\end{matrix}$where:g(t)=P _(Tc)(t)*h _(R)(t)  Equation (10)M is the number of multipath components. Here, h_(R)(t) is the impulseresponse of the chip matched filter 301 and denotes convolution. Theorder of the summations can be rewritten as:

$\begin{matrix}{{{x(t)} = {\sum\limits_{k = {- 8}}^{\infty}{c_{k}{f( {t - {k\; T_{c}}} )}}}}\text{where:}} & \text{Equation~~(11)} \\{{f(t)} = {\sum\limits_{i = 1}^{M}{a_{i}{g( {t - \tau_{i}} )}}}} & \text{Equation~~(12)}\end{matrix}$In the multipath channel described above, the sampler 302 samples theoutput signal of the chip matched filter 301 to produce x(nT) at themaximum power level points of g(t). In practice, however, the waveformg(t) is severely distorted because of the effect of the multipath signalreception, and a perfect time alignment of the signals is not available.

When the multipath distortion in the channel is negligible and a perfectestimate of the timing is available, i.e., a₁=1, t₁=0, and a₁=0, i=2, .. . , M, the received signal is r(t)=s(t). Then, with this ideal channelmodel, the output of the chip matched filter becomes:

$\begin{matrix}{{x(t)} = {\sum\limits_{k = {- \infty}}^{\infty}{c_{k}{g( {t - {k\; T_{c}}} )}}}} & \text{Equation~~(13)}\end{matrix}$

When there is multipath fading, however, the received spreading codewaveform is distorted, and has a number of local maxima that can changefrom one sampling interval to another depending on the channelcharacteristics. For multipath fading channels with quickly changingchannel characteristics, it is not practical to try to locate themaximum of the waveform f(t) in every chip period interval. Instead, atime reference may be obtained from the characteristics of f(t) that maynot change as quickly. Three tracking methods are described based ondifferent characteristics of f(t).

XIII. Prior Art Spreading Code Tracking Method

Prior art tracking methods include a code tracking circuit in which thereceiver attempts to determine the timing of the maximum matched filteroutput value of the chip waveform occurs and samples the signalaccordingly. However, in multipath fading channels, the receiverdespread code waveform can have a number of local maxima, especially ina mobile environment. In the following, f(t) represents the receivedsignal waveform of the spreading code chip convolved with the channelimpulse response. The frequency response characteristic of f(t) and themaximum of this characteristic can change rather quickly making itimpractical to track the maximum of f(t).

Define τ to be the time estimate that the tracking circuit calculatesduring a particular sampling interval. Also, define the following errorfunction as:

$\begin{matrix}\begin{matrix}{ɛ\{ \underset{{{\{{t:{{\tau - t}}}\rangle}\delta}\}}{\int{{f(t)}{\mathbb{d}t}}} } & { {{\tau - t}} \rangle\delta} \\{ɛ = 0} & {{{\tau - t}}\langle \delta }\end{matrix} & \text{Equation~~(14)}\end{matrix}$The tracking circuits of the prior art calculate a value of the inputsignal that minimizes the error ε. One can write:

$\begin{matrix}{{{\min\mspace{14mu} ɛ} = {1 - {\max\mspace{14mu}\tau{\int_{\tau - \delta}^{\tau + \delta}{f(t)}}}}}\;} & \text{Equation~~(15)}\end{matrix}$

Assuming f(τ) has a smooth shape in the values given, the value of τ forwhich f(τ) is maximum minimizes the error ε, so the tracking circuittracks the maximum point of f(t).

XIV. Median Weighted Value Tracking Method

The median weighted tracking method of one embodiment of the presentinvention, minimizes the absolute weighted error, defined as:

$\begin{matrix}{ɛ = {\int_{- \infty}^{\infty}{{{t - \tau}}{f(t)}{\mathbb{d}t}}}} & \text{Equation~~(16)}\end{matrix}$This tracking method calculates the ‘median’ signal value of f(τ) bycollecting information from all paths, where f(τ) is as in Equation(12). In a multipath fading environment, the waveform f(τ) can havemultiple local maxima, but only one median. To minimize ε, thederivative of Equation (16) is taken with respect to τ and the result isequated to zero, which provides:

$\begin{matrix}{{\int_{- \infty}^{\tau}{{f(t)}\ {\mathbb{d}t}}} = {\int_{\tau}^{\infty}{{f(t)}\ {\mathbb{d}t}}}} & \text{Equation~~(17)}\end{matrix}$The value of τ that satisfies Equation (17) is called the ‘median’ off(t). Therefore, the median tracking method of the present embodimenttracks the median of f(t).

FIG. 4 shows an implementation of the tracking circuit based onminimizing the absolute weighted error defined above. The signal x(t)and its one-half chip offset version x(t+T/2) are sampled by the A/D 401at a rate 1/T. The following Equation determines the operatingcharacteristic of the circuit in FIG. 4:

$\begin{matrix}{{ɛ\;\tau} = {{\sum\limits_{n = 1}^{2\; L}{{f( {\tau - {n\;{T/2}}} )}}} - {{f( {\tau + {n\;{T/2}}} )}}}} & \text{Equation~~(18)}\end{matrix}$

Tracking the median of a group of multipath signals keeps the receivedenergy of the multipath signal components substantially equal on theearly and late sides of the median point of the correct locallygenerated spreading code phase c_(n). The tracking circuit consists ofan A/D 401 which samples an input signal x(t) to form the half-chipoffset samples. The half chip offset samples are grouped into an earlyset of samples and a late set of samples. The first correlation bankadaptive matched filter 402 multiplies each early sample by thespreading code phases c(n+1), c(n+2), . . . , c(n+L), where L is smallcompared to the code length and approximately equal to half the numberof chips of delay between the earliest and latest multipath signal. Theoutput of each correlator is applied to a respective first sum-and-dumpbank 404. The magnitudes of the output values of the L sum-and-dumps arecalculated in the calculator 406 and then summed in summer 408 to givean output value proportional to the signal energy in the early multipathsignals. Similarly, a second correlation bank adaptive matched filter403 operates on the late samples, using code phases c(n−1), c(n−2), . .. , c(n−L), and each output signal is applied to a respectivesum-and-dump circuit in an integrator 405. The magnitudes of the Lsum-and-dump output signals are calculated in calculator 407 and thensummed in summer 409 to give a value for the late multipath signalenergy. Finally, the subtractor 410 calculates the difference andproduces error signal ε(t) of the early and late signal energy values.

The tracking circuit adjusts by means of error signal ε(t) the locallygenerated code phases c(t) to cause the difference between the early andlate values to tend toward 0.

XV. Centroid Tracking Method

The optimal spreading code tracking circuit of one embodiment of thepresent invention is called the squared weighted tracking (or centroid)circuit. Defining t to denote the time estimate that the trackingcircuit calculates, based on some characteristic of f(t), the centroidtracking circuit minimizes the squared weighted error defined as:

$\begin{matrix}{ɛ = {\int_{- \infty}^{\infty}{{{t - \tau}}^{2}{f(t)}\ {\mathbb{d}t}}}} & \text{Equation~~(19)}\end{matrix}$This function inside the integral has a quadratic form, which has aunique minimum. The value of t that minimizes ε can be found by takingthe derivative of the above Equation (19) with respect to t and equatingto zero, which gives:

$\begin{matrix}{{\int_{- \infty}^{\infty}{( {{{- 2}t} + {2\tau}} ){f(t)}{\mathbb{d}t}}} = 0} & \text{Equation~~(20)}\end{matrix}$Therefore, the value of t that satisfies Equation (21) is:

$\begin{matrix}{{\tau - {\frac{1}{\beta}{\int_{- \infty}^{\infty}{t\;{f(t)}{\mathbb{d}t}}}}} = 0} & \text{Equation~~(21)}\end{matrix}$is the timing estimate that the tracking circuit calculates, where β isa constant value.

Based on these observations, a realization of an exemplary trackingcircuit which minimizes the squared weighted error is shown in FIG. 5 a.The following Equation determines the error signal ε (t) of the centroidtracking circuit:

$\begin{matrix}{{ɛ(\tau)} = {{\sum\limits_{n = 1}^{2\; L}{n\lbrack {{{f( {\tau - {n\;{T/2}}} )}} - {{f( {\tau + {n\;{T/2}}} )}}} \rbrack}} = 0}} & \text{Equation~~(22)}\end{matrix}$The value that satisfies ε(t)=0 is the perfect estimate of the timing.

The early and late multipath signal energy on each side of the centroidpoint are equal. The centroid tracking circuit shown in FIG. 5 aconsists of an A/D converter 501 which samples an input signal x(t) toform the half-chip offset samples. The half chip offset samples aregrouped as an early set of samples and a late set of samples. The firstcorrelation bank adaptive matched filter 502 multiplies each earlysample and each late sample by the positive spreading code phasesc(n+1), c(n+2), . . . , c(n+L), where L is small compared to the codelength and approximately equal to half the number of chips of delaybetween the earliest and latest multipath signal. The output signal ofeach correlator is applied to a respective one of L sum-and-dumpcircuits of the first sum and dump bank 504. The magnitude value of eachsum-and-dump circuit of the sum and dump bank 504 is calculated by therespective calculator in the calculator bank 506 and applied to acorresponding weighting amplifier of the first weighting bank 508. Theoutput signal of each weighting amplifier represents the weighted signalenergy in a multipath component signal.

The weighted early multipath signal energy values are summed in sampleadder 510 to give an output value proportional to the signal energy inthe group of multipath signals corresponding to positive code phaseswhich are the early multipath signals. Similarly, a second correlationbank adaptive matched filter 503 operates on the late samples, using thenegative spreading code phases c(n−1), c(n−2), . . . , c(n−L); eachoutput signal is provided to a respective sum-and-dump circuit ofdiscrete integrator 505. The magnitude value of the L sum-and-dumpoutput signals are calculated by the respective calculator of calculatorbank 507 and then weighted in weighting bank 509. The weighted latemultipath signal energy values are summed in sample adder 511 to give anenergy value for the group of multipath signals corresponding to thenegative code phases which are the late multipath signals. Finally, theadder 512 calculates the difference of the early and late signal energyvalues to produce error sample value ε(t).

The tracking circuit of FIG. 5 a produces error signal ε(t) which isused to adjust the locally generated code phase c(nT) to keep theweighted average energy in the early and late multipath signal groupsequal. The embodiment shown uses weighting values that increase as thedistance from the centroid increases. The signal energy in the earliestand latest multipath signals is probably less than the multipath signalvalues near the centroid. Consequently, the difference calculated by theadder 510 is more sensitive to variations in delay of the earliest andlatest multipath signals.

XVI. Quadratic Detector for Tracking

In this embodiment of the tracking method, the tracking circuit adjuststhe sampling phase to be “optimal” and robust to multipath. Let f(t)represent the received signal waveform as in Equation (12) above. Theparticular method of optimizing starts with a delay locked loop with anerror signal ε(t) that drives the loop. The function ε(t) must have onlyone zero at τ=τ₀ where τ₀ is optimal. The optimal form for ε(τ) has thecanonical form:

$\begin{matrix}{{ɛ(\tau)} = {\int_{- \infty}^{\infty}{{w( {t,\tau} )}{{f(t)}}^{2}{\mathbb{d}t}}}} & \text{Equation~~(23)}\end{matrix}$where w(t,τ) is a weighting function relating f(t) to the error ε(τ),and the relationship indicated by Equation (24) also holds:

$\begin{matrix}{{ɛ( {\tau + \tau_{0}} )} = {\int_{- \infty}^{\infty}{{w( {t,{\tau + \tau_{0}}} )}{{f(t)}}^{2}{\mathbb{d}t}}}} & \text{Equation~~(24)}\end{matrix}$

It follows from Equation (24) that w(t,τ) is equivalent to w(t−τ).Considering the slope M of the error signal in the neighborhood of alock point τ₀:

$\begin{matrix}{M = {\frac{\mathbb{d}{ɛ(\tau)}}{\mathbb{d}\tau}{_{\tau\; 0}{= {- {\int_{- \infty}^{\infty}{{w^{\prime}( {t - \tau_{0}} )}{g(t)}{\mathbb{d}t}}}}}}}} & \text{Equation~~(25)}\end{matrix}$where w′(t, τ) is the derivative of w(t, τ) with respect to τ, and g(t)is the average of |f(t)|².

The error ε(t) has a deterministic part and a noise part. Let z denotethe noise component in ε(t), then |z|² is the average noise power in theerror function ε(t). Consequently, the optimal tracking circuitmaximizes the ratio

$\begin{matrix}{F = \frac{M^{2}}{{z}^{2}}} & \text{Equation~~(26)}\end{matrix}$

The implementation of the quadratic detector is now described. Thediscrete error value ε of an error signal ε(t) is generated byperforming the operationε=y^(T)By  Equation (27)where the vector y represents the received signal components yi, i=0, 1,. . . L−1, as shown in FIG. 5 b. The matrix B is an L by L matrix andthe elements are determined by calculating values such that the ratio Fof Equation (26) is maximized. The quadratic detector described abovemay be used to implement the centroid tracking system described abovewith reference to FIG. 5 a. For this implementation, the vector y is theoutput signal of the sum and dump circuits 504: y={f(τ−LT), f(τ−LT+T/2),f(τ−(L−1)T), . . . f(τ), f(τ+T/2), f(τ+T), . . . f(τ+LT)} and the matrixB is set forth in Table 9.

TABLE 9 B matrix for quadratic form of Centroid Tracking System L 0 0 00 0 0 0 0 0 0 0 L − ½ 0 0 0 0 0 0 0 0 0 0 0 L − 1 0 0 0 0 0 0 0 0 . . ..  . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . 0 0 0 0 ½0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −½ 0 0 0 0 . . . . . . ..  . . . . . . . . . . . . . . . . . . . . .  . . . . 0 0 0 0 0 0 0 0−L + 1 0 0 0 0 0 0 0 0 0 0 0 −L + ½ 0 0 0 0 0 0 0 0 0 0 0 −L

XVII. Determining the Minimum Value of L Needed

The value of L in the previous section determines the minimum number ofcorrelators and sum-and-dump elements. L is chosen as small as possiblewithout compromising the functionality of the tracking circuit.

The multipath characteristic of the channel is such that the receivedchip waveform f(t) is spread over QT_(c) seconds, or the multipathcomponents occupy a time period of Q chips duration. The value of Lchosen is L=Q. Q is found by measuring the particular RF channeltransmission characteristics to determine the earliest and latestmultipath component signal propagation delay. QT_(c) is the differencebetween the earliest and latest multipath component arrival time at areceiver.

XVIII. Adaptive Vector Correlator

An embodiment of the present invention uses an adaptive vectorcorrelator (AVC) to estimate the channel impulse response and to obtaina reference value for coherent combining of received multipath signalcomponents. The described embodiment employs an array of correlators toestimate the complex channel response affecting each multipathcomponent. The receiver compensates for the channel response andcoherently combines the received multipath signal components. Thisapproach is referred to as maximal ratio combining.

Referring to FIG. 6, the input signal x(t) to the system includesinterference noise of other message channels, multipath signals of themessage channels, thermal noise, and multipath signals of the pilotsignal. The signal is provided to AVC 601 which, in the exemplaryembodiment, includes a despreading means 602, channel estimation meansfor estimating the channel response 604, correction means for correctinga signal for effects of the channel response 603 and adder 605. The AVCdespreading means 602 is composed of multiple code correlators, witheach correlator using a different phase of the pilot code c(t) providedby the pilot code generator 608. The output signal of this despreadingmeans corresponds to a noise power level if the local pilot code of thedespreading means is not in phase with the input code signal.Alternatively, it corresponds to a received pilot signal power levelplus noise power level if the phases of the input pilot code and locallygenerated pilot code are the same. The output signals of the correlatorsof the despreading means are corrected for the channel response by thecorrection means 603 and are applied to the adder 605 which collects allmultipath pilot signal power. The channel response estimation means 604receives the combined pilot signal and the output signals of thedespreading means 602, and provides a channel response estimate signal,w(t), to the correction means 603 of the AVC, and the estimate signalw(t) is also available to the adaptive matched filter (AMF) describedbelow. The output signal of the despreading means 602 is also providedto the acquisition decision means 606 which decides, based on aparticular algorithm such as a sequential probability ratio test (SPRT),if the present output levels of the despreading circuits correspond tosynchronization of the locally generated spreading code to the desiredinput code phase. If the detector finds no synchronization, then theacquisition decision means sends a control signal a(t) to the localpilot code generator 608 to offset its phase by one or more chip period.When synchronization is found, the acquisition decision means informstracking circuit 607, which achieves and maintains a closesynchronization between the received and locally generated spreadingcodes.

An exemplary implementation of the pilot AVC used to despread the pilotspreading code is shown in FIG. 7. The described embodiment assumes thatthe input signal x(τ) has been sampled with sampling period T to formsamples x(nT+τ), and is composed of interference noise of other messagechannels, multipath signals of message channels, thermal noise andmultipath signals of the pilot code. The signal x(nT+τ) is applied to Lcorrelators, where L is the number of code phases over which theuncertainty within the multipath signals exists. Each correlator 701,702, 703 comprises a multiplier 704, 705, 706, which multiples the inputsignal with a particular phase of the pilot spreading code signalc((n+i)T) and sum-and-dump circuits 708, 709, 710. The output signal ofeach multiplier 704, 705, 706 is applied to a respective sum- and dumpcircuit 708, 709, 710 to perform discrete integration. Before summingthe signal energy contained in the outputs of the correlators, the AVCcompensates for the channel response and the carrier phase rotation ofthe different multipath signals. Each output of each sum-and-dump 708,709, 710 is multiplied with a derotation phaser [complex conjugate ofep(nT)] from digital phase lock loop (DPLL) 721 by the respectivemultiplier 714, 715, 716 to account for the phase and frequency offsetof the carrier signal. The pilot rake AMF calculates the weightingfactors wk, k=1, . . . , L, for each multipath signal by passing theoutput of each multiplier 714, 715, 716 through a low pass filter (LPF)711, 712, 713. Each despread multipath signal is multiplied by itscorresponding weighting factor in a respective multiplier 717, 718, 719.The output signals of the multipliers 717, 718, 719 are summed in amaster adder 720, and the output signal p(nT) of the accumulator 720consists of the combined despread multipath pilot signals in noise. Theoutput signal p(nT) is also input to the DPLL 721 to produce the errorsignal ep(nT) for tracking of the carrier phase.

FIGS. 8 a and 8 b show alternate embodiments of the AVC which can beused for detection and multipath signal component combining. The messagesignal AVCs of FIGS. 8 a and 8 b use the weighting factors produced bythe pilot AVC to correct the message data multipath signals. Thespreading code signal, c(nT) is the spreading code spreading sequenceused by a particular message channel and is synchronous with the pilotspreading code signal. The value L is the number of correlators in theAVC circuit.

The circuit of FIG. 8 a calculates the decision variable Z which isgiven by:

$\begin{matrix}{Z = {{w_{1}{\sum\limits_{i = 1}^{N}{{x( {{i\; T} + \tau} )}{c( {i\; T} )}}}} + {w_{2}{\sum\limits_{i = 1}^{N}{{x( {{i\; T} + \tau} )}{c( {( {i + 1} )T} )}}}} + {\quad{\ldots + {W_{L}{\sum\limits_{i = 1}^{L}{x( {{i\; T} + \tau} )}}} + {c( {( {i + L} )T} )}}}}} & {{Equation}\mspace{14mu}(28)}\end{matrix}$where N is the number of chips in the correlation window. Equivalently,the decision statistic is given by:

$\begin{matrix}\begin{matrix}{Z = {{{x( {T + \tau} )}{\sum\limits_{i = 1}^{L}{w_{1}{c( {i\; T} )}}}} + {{x( {{2\; T} + \tau} )}{\sum\limits_{i = 1}^{L}{w_{1}{c( {( {i + 1} )T} )}}}} + \ldots +}} \\{{x( {{N\; T} + \tau} )}{\sum\limits_{i = 1}^{L}{w_{N}{c( {( {i + N} )T} )}}}} \\{= {\sum\limits_{k = 1}^{N}{{x( {{k\; T} - \tau} )}{\sum\limits_{i = 1}^{L}{w_{k}{c( {( {i + k - 1} )T} )}}}}}}\end{matrix} & {{Equation}\mspace{14mu}(29)}\end{matrix}$The alternative implementation that results from Equation (29) is shownin FIG. 8 b.

Referring to FIG. 8 a, the input signal x(t) is sampled to form x(nT+t),and is composed of interference noise of other message channels,multipath signals of message channels, thermal noise, and multipathsignals of the pilot code. The signal x(nT+t) is applied to Lcorrelators, where L is the number of code phases over which theuncertainty within the multipath signals exists. Each correlator 801,802, 803 comprises a multiplier 804, 805, 806, which multiples the inputsignal by a particular phase of the message channel spreading codesignal, and a respective sum-and-dump circuit 808, 809, 810. The outputsignal of each multiplier 804, 805, 806 is applied to a respective sum-and dump circuit 808, 809, 810 which performs discrete integration.Before summing the signal energy contained in the output signals of thecorrelators, the AVC compensates for the different multipath signals.Each despread multipath signal and its corresponding weighting factor,which is obtained from the corresponding multipath weighting factor ofthe pilot AVC, are multiplied in a respective multiplier 817, 818, 819.The output signals of multipliers 817, 818, 819 are summed in a masteradder 820, and the output signal z(nT) of the accumulator 820 consistsof sampled levels of a despread message signal in noise.

The alternative embodiment of the invention includes a newimplementation of the AVC despreading circuit for the message channelswhich performs the sum-and-dump for each multipath signal componentsimultaneously. The advantage of this circuit is that only one sum- anddump circuit and one adder is necessary. Referring to FIG. 8 b, themessage code sequence generator 830 provides a message code sequence toshift register 831 of length L. The output signal of each register 832,833, 834, 835 of the shift register 831 corresponds to the message codesequence shifted in phase by one chip. The output value of each register832, 833, 834, 835 is multiplied in multipliers 836, 837, 838, 839 withthe corresponding weighting factor w_(k), k=1, . . . , L obtained fromthe pilot AVC. The output signals of the L multipliers 836, 837, 838,839 are summed by the adding circuit 840. The adding circuit outputsignal and the receiver input signal x(nT+τ) are then multiplied in themultiplier 841 and integrated by the sum-and-dump circuit 842 to producemessage signal z(nT).

A third embodiment of the adaptive vector correlator is shown in FIG. 8c. The embodiment shown uses the least mean square (LMS) statistic toimplement the vector correlator and determines the decoration factorsfor each multipath component from the received multipath signal. The AVCof FIG. 8 c is similar to the exemplary implementation of the Pilot AVCused to despread the pilot spreading code shown in FIG. 7. The digitalphase locked loop 721 is replaced by the phase locked loop 850 havingvoltage controlled oscillator 851, loop filter 852, limiter 853 andimaginary component separator 854. The difference between the correcteddespread output signal ido and an ideal despread output signal isprovided by adder 855, and the difference signal is a despread errorvalue ide which is further used by the decoration circuits to compensatefor errors in the decoration factors.

In a multipath signal environment, the signal energy of a transmittedsymbol is spread out over the multipath signal components. The advantageof multipath signal addition is that a substantial portion of signalenergy is recovered in an output signal from the AVC. Consequently, adetection circuit has an input signal from the AVC with a highersignal-to-noise ratio (SNR), and so can detect the presence of a symbolwith a lower bit-error ratio (BER). In addition, measuring the output ofthe AVC is a good indication of the transmit power of the transmitter,and a good measure of the system's interference noise.

XIX. Adaptive Matched Filter

One embodiment of the current invention includes an adaptive matchedfilter (AMF) to optimally combine the multipath signal components in areceived spread spectrum message signal. The AMF is a tapped delay linewhich holds shifted values of the sampled message signal and combinesthese after correcting for the channel response. The correction for thechannel response is done using the channel response estimate calculatedin the AVC which operates on the pilot sequence signal. The outputsignal of the AMF is the combination of the multipath components whichare summed to give a maximum value. This combination corrects for thedistortion of multipath signal reception. The various messagedespreading circuits operate on this combined multipath component signalfrom the AMF. FIG. 8 d shows an exemplary embodiment of the AMF. Thesampled signal from the A/D converter 870 is applied to the L-stagedelay line 872. Each stage of this delay line 872 holds the signalcorresponding to a different multipath signal component. Correction forthe channel response is applied to each delayed signal component bymultiplying the component in the respective multiplier of multiplierbank 874 with the respective weighting factor w₁, w₂, . . . , w_(L) fromthe AVC corresponding to the delayed signal component. All weightedsignal components are summed in the adder 876 to give the combinedmultipath component signal y(t).

The combined multipath component signal y(t) does not include thecorrection due to phase and frequency offset of the carrier signal. Thecorrection for the phase and frequency offset of the carrier signal ismade to y(t) by multiplying y(t) with carrier phase and frequencycorrection (derotation phaser) in multiplier 878. The phase andfrequency correction is produced by the AVC as described previously.FIG. 8 d shows the correction as being applied before the despreadingcircuits 880, but alternate embodiments of the invention can apply thecorrection after the despreading circuits.

XX. Method to Reduce Re-Acquisition Time with Virtual Location

One consequence of determining the difference in code phase between thelocally generated pilot code sequence and a received spreading codesequence is that an approximate value for the distance between the basestation and a subscriber unit can be calculated. If the SU has arelatively fixed position with respect to the RCS of the base station,the uncertainty of received spreading code phase is reduced forsubsequent attempts at re-acquisition by the SU or RCS. The timerequired for the base station to acquire the access signal of a SU thathas gone “off-hook” contributes to the delay between the SU goingoff-hook and the receipt of a dial tone from the PSTN. For systems thatrequire a short delay, such as 150 msec for dial tone after off-hook isdetected, a method which reduces the acquisition and bearer channelestablishment time is desirable. One embodiment of the present inventionuses such a method of reducing re-acquisition by use of virtuallocating. Additional details of this technique are described in SectionXXXXIII hereinafter entitled “Virtual Locating Of A Fixed SubscriberUnit To Reduce Re-Acquisition Time”.

The RCS acquires the SU CDMA signal by searching only those receivedcode phases corresponding to the largest propagation delay of theparticular system. In other words, the RCS assumes that all SUs are at apredetermined, fixed distance from the RCS. The first time the SUestablishes a channel with the RCS, the normal search pattern isperformed by the RCS to acquire the access channel. The normal methodstarts by searching the code phases corresponding to the longestpossible delay, and gradually adjusts the search to the code phases withthe shortest possible delay. However, after the initial acquisition, theSU can calculate the delay between the RCS and the SU by measuring thetime difference between sending a short access signal to the RCS andreceiving an acknowledgment signal, and using the received global pilotchannel as a timing reference. The SU can also receive the delay valueby having the RCS calculate the round trip delay difference from thecode phase difference between the global pilot code generated at the RCSand the received assigned pilot code from the SU, and then sending theSU the value on a predetermined control channel. Once the round tripdelay is known to the SU, the SU may adjust the code phase of thelocally generated assigned pilot and spreading codes by adding the delayrequired to make the SU appear to the RCS to be at the predeterminedfixed distance from the RCS. Although the method is explained for thelargest delay, a delay corresponding to any predetermined location inthe system can be used.

A second advantage of the method of reducing re-acquisition by virtuallocating is that a conservation in SU power use can be achieved. Notethat a SU that is “powered down” or in a sleep mode needs to start thebearer channel acquisition process with a low transmit power level andramp-up power until the RCS can receive its signal in order to minimizeinterference with other users. Since the subsequent re-acquisition timeis shorter, and because the SU's location is relatively fixed inrelation to the RCS, the SU can ramp-up transmit power more quicklybecause the SU will wait a shorter period of time before increasingtransmit power. The SU waits a shorter period because it knows, within asmall error range, when it should receive a response from the RCS if theRCS has acquired the SU signal.)

XXI. The Radio Carrier Station (RCS)

The Radio Carrier Station (RCS) of the present invention acts as acentral interface between the SU and the remote processing controlnetwork element, such as a radio distribution unit (RDU). The interfaceto the RDU of the present embodiment follows the G.704 standard and aninterface according to a modified version of DECT V5.1, but the presentinvention can support any interface that can exchange call control andtraffic channels. The RCS receives information channels from the RDUincluding call control data, and traffic channel data such as, but notlimited to, 32 kbs ADPCM, 64 kbs PCM and ISDN, as well as systemconfiguration and maintenance data. The RCS also terminates the CDMAradio interface bearer channels with SUs, which channels include bothcontrol data, and traffic channel data. In response to the call controldata from either the RDU or a SU, the RCS allocates traffic channels tobearer channels on the RF communication link and establishes acommunication connection between the SU and the telephone networkthrough an RDU.

As shown in FIG. 9, the RCS receives call control and messageinformation data into the MUXs 905, 906 and 907 through interface lines901, 902 and 903. Although E1 format is shown, other similartelecommunication formats can be supported in the same manner asdescribed below. The MUXs shown in FIG. 9 may be implemented usingcircuits similar to that shown in FIG. 10. The MUX shown in FIG. 10includes system clock signal generator 1001 consisting of phase lockedoscillators (not shown) which generate clock signals for the line PCMhighway 1002 (which is part of PCM highway 910), and high speed bus(HSB) 970; and the MUX controller 1010 which synchronizes the systemclock 1001 to interface line 1004. It is contemplated that the phaselock oscillators can provide timing signals for the RCS in the absenceof synchronization to a line. The MUX line interface 1011 separates thecall control data from the message information data. Referring to FIG.9, each MUX provides a connection to the wireless access controller(WAC) 920 through the PCM highway 910. The MUX controller 1010 alsomonitors the presence of different tones present in the informationsignal by means of tone detector 1030. Additionally, the MUX Controller1010 provides the ISDN D channel network signaling locally to the RDU.

The MUX line interface 1011, such as a FALC 54, includes an E1 interface1012 which consists of a transmit connection pair (not shown) and areceive connection pair (not shown) of the MUX connected to the RDU orcentral office (CO) ISDN switch at the data rate of 2.048 Mbps. Thetransmit and receive connection pairs are connected to the E1 interface1012 which translates differential tri-level transmit/receive encodedpairs into levels for use by the framer 1015. The line interface 1011uses internal phase-locked-loops (not shown) to produce E1-derived 2.048MHz and 4.096 MHz clocks as well as an 8 KHz frame-sync pulse. The lineinterface can operate in clock-master or clock-slave mode. While theexemplary embodiment is shown as using an E1 interface, it iscontemplated that other types of telephone lines which convey multiplecalls may be used, for example, T1 lines or lines which interface to aprivate branch exchange (PBX).

The line interface framer 1015 frames the data streams by recognizingthe framing patterns on channel-1 (time-slot 0) of the incoming line,inserts and extracts service bits and generates/checks line servicequality information.

As long as a valid E1 signal appears at the E1 interface 1012, the FALC54, recovers a 2.048 MHz PCM clock signal from the E1 line. This clock,via system clock 1001, is used system wide as a PCM highway clocksignal. If the E1 line fails, the FALC 54 continues to deliver a PCMclock derived from an oscillator signal o(t) connected to the sync input(not shown) of the FALC 54. This PCM clock serves the RCS system untilanother MUX with an operational E1 line assumes responsibility forgenerating the system clock signals.

The framer 1015 generates a received frame sync pulse, which in turn canbe used to trigger the PCM Interface 1016 to transfer data onto the linePCM highway 1002 and into the RCS system for use by other elements.Since all E1 lines are frame synchronized, all line PCM highways arealso frame synchronized. From this 8 kHz PCM Sync pulse, the systemclock signal generator 1001 of the MUX uses a phase locked loop (notshown) to synthesize the PN×2 clock (e.g., 15.96 MHz)(W₀(t)). Thefrequency of this clock signal is different for different transmissionbandwidths as described in Table 7.

The MUX includes a MUX controller 1010, such as a 25 MHz quad integratedcommunications controller, containing a microprocessor 1020, programmemory 1021, and time division multiplexer (TDM) 1022. The TDM 1022 iscoupled to receive the signal provided by the framer 1015, and extractsinformation placed in time slots 0 and 16. The extracted informationgoverns how the MUX controller 1010 processes the link access protocol—D(LAPD) data link. The call control and bearer modification messages,such as those defined as V5.1 network layer messages, are either passedto the WAC, or used locally by the MUX controller 1010.

The RCS line PCM highway 1002 is connected to and originates with theframer 1015 through PCM Interface 1016, and is comprised of a 2.048 MHzstream of data in both the transmit and receive direction. The RCS alsocontains a high speed bus (HSB) 970 which is the communication linkbetween the MUX, WAC, and MIUs. The HSB 970 supports a data rate of, forexample, 100 Mbit/sec. Each of the MUX, WAC, and MIU access the HSBusing arbitration. The RCS of the present invention also can includeseveral MUXs requiring one board to be a “master” and the rest “slaves”.Details on the implementation of the HSB may be found in Section XXXXIVhereinafter entitled PARALLEL PACKETIZED INTERMODULE ARBITRATED HIGHSPEED CONTROL AND DATA BUS.

Referring to FIG. 9, the wireless access controller (WAC) 920 is the RCSsystem controller which manages call control functions andinterconnection of data streams between the MUXs 905, 906, 907, modeminterface units (MIUs) 931, 932, 933. The WAC 920 also controls andmonitors other RCS elements such as the VDC 940, RF 950, and poweramplifiers 960. The WAC 920 as shown in FIG. 11, allocates bearerchannels to the modems on each MIU 931, 932, 933 and allocates themessage data on line PCM Highway 910 from the MUXs 905, 906, 907 to themodems on the MIUs 931, 932, 933. This allocation is made through theSystem PCM Highway 911 by means of a time slot interchange on the WAC920. If more than one WAC is present for redundancy purposes, the WACsdetermine the master-slave relationship with a second WAC. The WAC 920also generates messages and paging information responsive to callcontrol signals from the MUXs 905, 906, 907 received from a remoteprocessor, such as an RDU; generates broadcast data which is transmittedto the MIU master modem 934; and controls the generation by the MIU MM934 of the Global system Pilot spreading code sequence. The WAC 920 alsois connected to an external network manager (NM) 980 for craftsperson oruser access.

Referring to FIG. 11, the WAC includes a time-slot interchanger (TSI)1101 which transfers information from one time slot in a line PCMhighway or system PCM highway to another time slot in either the same ordifferent line PCM highway or system PCM highway. The TSI 1101 isconnected to the WAC controller 1111 of FIG. 11 which controls theassignment or transfer of information from one time slot to another timeslot and stores this information in memory 1120. The exemplaryembodiment of the invention has four PCM Highways 1102, 1103, 1104, 1105connected to the TSI. The WAC also is connected to the HSB 970, throughwhich WAC communicates to a second WAC (not shown), to the MUXs and tothe MIUs.

Referring to FIG. 11, the WAC 920 includes a WAC controller 1111employing, for example, a microprocessor 1112, such as a Motorola MC68040 and a communications processor 1113, such as the Motorola MC68360QUICC communications processor, and a clock oscillator 1114 whichreceives a clock synch signal wo(t) from the system clock generator. Theclock generator is located on a MUX (not shown) to provide timing to theWAC controller 1111. The WAC controller 1111 also includes memory 1120including flash PROM 1121 and SRAM memory 1122. The flash PROM 1121contains the program code for the WAC controller 1111 and isreprogrammable for new software programs downloaded from an externalsource. The SRAM 1122 is provided to contain the temporary data writtento and read from memory 1120 by the WAC controller 1111.

A low speed bus 912 is connected to the WAC 920 for transferring controland status signals between the RF transmitter/receiver 950, VDC 940, RF950 and power amplifier 960 as shown in FIG. 9. The control signals aresent from the WAC 920 to enable or disable the RF transmitters/receiver950 or power amplifier 960, and the status signals are sent from the RFtransmitters/receiver 950 or power amplifier 960 to monitor the presenceof a fault condition.

The exemplary RCS contains at least one MIU 931, which is shown in FIG.12 and now described in detail. The MIU of the exemplary embodimentincludes six CDMA modems, but the invention is not limited to thisnumber of modems. The MIU includes a system PCM highway 1201 connectedto each of the CDMA Modems 1210, 1211, 1212, 1215 through a PCMInterface 1220, a control channel bus 1221 connected to MIU controller1230 and each of the CDMA modems 1210, 1211, 1212, 1215, an MIU clocksignal generator (CLK) 1231, and a modem output combiner 1232. The MIUprovides the RCS with the following functions: the MIU controllerreceives CDMA channel assignment instructions from the WAC and assigns amodem to a user information signal which is applied to the lineinterface of the MUX and a modem to receive the CDMA channel from theSU; it also combines the CDMA transmit modem data for each of the MIUCDMA modems; multiplexes I and Q transmit message data from the CDMAmodems for transmission to the VDC; receives analog I and Q receivemessage data from the VDC; distributes the I and Q data to the CDMAmodems; transmits and receives digital AGC data; distributes the AGCdata to the CDMA modems; and sends MIU board status and maintenanceinformation to the WAC 920.

The MIU controller 1230 of the exemplary embodiment of the presentinvention contains one communication microprocessor 1240, such as theMC68360 “QUICC” processor, and includes a memory 1242 having a FlashPROM memory 1243 and a SRAM memory 1244. Flash PROM 1243 is provided tocontain the program code for the microprocessors 1240, and the memory1243 is downloadable and reprogrammable to support new program versions.SRAM 1244 is provided to contain the temporary data space needed by theMC68360 microprocessor 1240 when the MIU controller 1230 reads or writesdata to memory

The MIU clock signal generator (CLK) 1231 provides a timing signal tothe MIU controller 1230, and also provides a timing signal to the CDMAmodems 1210, 1211, 1212 and 1215. The MIU clock signal generator 1231receives, and is synchronized to, the system clock signal wo(t). The MIUclock signal generator 1231 also receives and synchronizes to thespreading code clock signal pn(t) which is distributed to the CDMAmodems 1210, 1211, 1212, 1215 from the MUX.

The RCS of the present embodiment includes a system modem 1210 containedon one MIU. The system modem 1210 includes a broadcast spreader (notshown) and a pilot generator (not shown). The broadcast modem providesthe broadcast information used by the exemplary system, and thebroadcast message data is transferred from the MIU controller 1230 tothe system modem 1210. The system modem also includes four additionalmodems (not shown) which are used to transmit the signals CT1 throughCT4 and AX1 through AX4. The system modem 1210 provides unweighted I andQ broadcast message data signals which are applied to the VDC. The VDCadds the broadcast message data signal to the MIU CDMA modem transmitdata of all CDMA modems 1210, 1211, 1212, 1215 and the global pilotsignal.

The pilot generator (PG) 1250 provides the global pilot signal which isused by the present invention, and the global pilot signal is providedto the CDMA modems 1210, 1211, 1212, 1215 by the MIU controller 1230.However, other embodiments of the present invention do not require theMIU controller to generate the global pilot signal, but include a globalpilot signal generated by any form of CDMA spreading code generator. Inthe described embodiment of the invention, the unweighted I and Q globalpilot signal is also sent to the VDC where it is assigned a weight, andadded to the MIU CDMA modem transmit data and broadcast message datasignal.

System timing in the RCS is derived from the E1 interface. There arefour MUXs in an RCS, three of which (905, 906 and 907) are shown in FIG.9. Two MUXs are located on each chassis. One of the two MUXs on eachchassis is designated as the master, and one of the masters isdesignated as the system master. The MUX which is the system masterderives a 2.048 MHz PCM clock signal from the E1 interface using aphase-locked loop (not shown). In turn, the system master MUX dividesthe 2.048 MHz PCM clock signal in frequency by 16 to derive a 128 KHzreference clock signal. The 128 KHz reference clock signal isdistributed from the MUX that is the system master to all the otherMUXs. In turn, each MUX multiplies the 128 KHz reference clock signal infrequency to synthesize the system clock signal which has a frequencythat is twice the frequency of the PN-clock signal. The MUX also dividesthe 128 KHz clock signal in frequency by 16 to generate the 8 KHz framesynch signal which is distributed to the MIUs. The system clock signalfor the exemplary embodiment has a frequency of 11.648 MHz for a 7 MHzbandwidth CDMA channel Each MUX also divides the system clock signal infrequency by 2 to obtain the PN-clock signal and further divides thePN-clock signal in frequency by 29 877 120 (the PN sequence length) togenerate the PN-synch signal which indicates the epoch boundaries. ThePN-synch signal from the system master MUX is also distributed to allMUXs to maintain phase alignment of the internally generated clocksignals for each MUX. The PN-synch signal and the frame synch signal arealigned. The two MUXs that are designated as the master MUXs for eachchassis then distribute both the system clock signal and the PN-clocksignal to the MIUs and the VDC.

The PCM highway interface 1220 connects the system PCM highway 911 toeach CDMA modem 1210, 1211, 1212, 1215. The WAC controller transmitsmodem control information, including traffic message control signals foreach respective user information signal to the MIU controller 1230through the HSB 970. Each CDMA modem 1210, 1211, 1212, 1215 receives atraffic message control signal, which includes signaling information,from the MIU controller 1111. Traffic message control signals alsoinclude call control (CC) information and spreading code and despreadingcode sequence information.

The MIU also includes the transmit data combiner 1232 which addsweighted CDMA modem transmit data including in-phase (I) and quadrature(Q) modem transmit data from the CDMA modems 1210, 1211, 1212, 1215 onthe MIU. The I modem transmit data is added separately from the Q modemtransmit data. The combined I and Q modem transmit data output signal ofthe transmit data combiner 1232 is applied to the I and Q multiplexer1233 that creates a single CDMA transmit message channel composed of theI and Q modem transmit data multiplexed into a digital data stream.

The receiver data input Circuit (RDI) 1234 receives the analogdifferential I and Q Data from the video distribution circuit (VDC) 940shown in FIG. 9 and distributes analog differential I and Q data to eachof the CDMA modems 1210, 1211, 1212, 1215 of the MIU. The AGCdistribution circuit 1235 receives the AGC data signal from the VDC anddistributes the AGC data to each of the CDMA modems of the MIU. The TRLcircuit 1233 receives the traffic lights information and similarlydistributes the Traffic light data to each of the Modems 1210, 1211,1212, 1215.

XXII. The CDMA Modem

The CDMA modem provides for generation of CDMA spreading codes andsynchronization between transmitter and receiver. It also provides fourfull duplex channels (TR0, TR1, TR2, TR3) programmable to 64, 32, 16,and 8 ksym/sec. each, for spreading and transmission at a specific powerlevel. The CDMA modem measures the received signal strength to allowAPC, it generates and transmits pilot signals, and encodes and decodesusing the signal for forward error correction (FEC). The modem in an SUalso performs transmitter spreading code pulse shaping using an FIRfilter. The CDMA modem is also used by the subscriber unit (SU), and inthe following discussion those features which are used only by the SUare distinctly pointed out. The operating frequencies of the CDMA modemare given in Table 10.

TABLE 10 Operating Frequencies Bandwidth Chip Rate Symbol Rate Gain(MHz) (MHz) (KHz) (Chips/Symbol) 7 5.824 64  91 10 8.320 64 130 10.58.512 64 133 14 11.648  64 182 15 12.480  64 195

Each CDMA modem 1210, 1211, 1212, 1215 of FIG. 12, and as shown in FIG.13, is composed of a transmit section 1301 and a receive section 1302.Also included in the CDMA modem is a control center 1303 which receivescontrol messages CNTRL from the external system. These messages areused, for example, to assign particular spreading codes, activate thespreading or despreading or to assign transmission rates. In addition,the CDMA modem has a code generator means 1304 used to generate thevarious spreading and despreading codes used by the CDMA modem. Thetransmit section 1301 is for transmitting the input information andcontrol signals m_(i)(t), i=1, 2, . . . I as spread-spectrum processeduser information signals sc_(j)(t), j=1, 2, . . . J. The transmitsection 1301 receives the global pilot code from the code generator 1304which is controlled by the control means 1303. The spread spectrumprocessed user information signals are ultimately added to other similarprocessed signals and transmitted as CDMA channels over the CDMA RFforward message link, for example to the SUs. The receive section 1302receives CDMA channels as r(t) and despreads and recovers the userinformation and control signals rc_(k)(t), k=1, 2, . . . K transmittedover the CDMA RF reverse message link, for example to the RCS from theSUs.

XXIII. CDMA Modem Transmitter Section

Referring to FIG. 14, the code generator means 1304 includes transmittiming control logic 1401 and spreading code PN-generator 1402, and thetransmit section 1301 includes modem input signal receiver (MISR) 1410,convolution encoders 1411, 1412, 1413, 1414, spreaders 1420, 1421, 1422,1423, 1424 and combiner 1430. The transmit section 1301 receives themessage data channels MESSAGE, convolutionally encodes each message datachannel in the respective convolutional encoder 1411, 1412, 1413, 1414,modulates the data with random spreading code sequence in the respectivespreader 1420, 1421, 1422, 1423, 1424, and combines modulated data fromall channels, including the pilot code received in the describedembodiment from the code generator, in the combiner 1430 to generate Iand Q components for RF transmission. The transmitter section 1301 ofthe present embodiment supports four (TR0, TR1, TR2, TR3) 64, 32, 16, 8kbs programmable channels. The message channel data is a timemultiplexed signal received from the PCM highway 1201 through PCMinterface 1220 and input to the MISR 1410.

FIG. 15 is a block diagram of an exemplary MISR 1410. For the exemplaryembodiment of the present invention, a counter is set by the 8 KHz framesynchronization signal MPCMSYNC and is incremented by 2.048 MHz MPCMCLKfrom the timing circuit 1401. The counter output is compared bycomparator 1502 against TRCFG values corresponding to slot time locationfor TR0, TR1, TR2, TR3 message channel data; and the TRCFG values arereceived from the MIU controller 1230. The comparator sends count signalto the registers 1505, 1506, 1507 and 1508 which clocks message channeldata into buffers 1510, 1511, 1512, 1513 using the TXPNCLK timing signalderived from the system clock. The message data is provided from thesignal MSGDAT from the PCM highway signal MESSAGE when enable signalsTR0EN, TR1EN, TR2EN and TR3EN from timing control logic 1401 are active.In further embodiments, MESSAGE may also include signals that enableregisters depending upon an encryption rate or data rate. If the counteroutput is equal to one of the channel location addresses, the specifiedtransmit message data in registers 1510, 1511, 1512, 1513 are input tothe convolutional encoders 1411, 1412, 1413, 1414 shown in FIG. 14.

The convolutional encoder enables the use of forward error correction(FEC) techniques, which are well known in the art. FEC techniques dependon introducing redundancy in generation of data in encoded form. Encodeddata is transmitted and the redundancy in the data enables the receiverdecoder device to detect and correct errors. One embodiment of thepresent invention employs convolutional encoding. Additional data bitsare added to the data in the encoding process and are the codingoverhead. The coding rate is expressed as the ratio of data bitstransmitted to the total bits (code data+redundant data) transmitted andis called the rate “R” of the code.

Convolution codes are codes where each code bit is generated by theconvolution of each new uncoded bit with a number of previously codedbits. The total number of bits used in the encoding process is referredto as the constraint length (K) of the code. In convolutional coding,data is clocked into a shift register of K bits length so that anincoming bit is clocked into the register, and it and the existing K−1bits are convolutionally encoded to create a new symbol. The convolutionprocess consists of creating a symbol consisting of a modulo-2 sum of acertain pattern of available bits, always including the first bit andthe last bit in at least one of the symbols.

FIG. 16 shows the block diagram of a K=7, R=½ convolution encodersuitable for use as the encoder 1411 shown in FIG. 14. This circuitencodes the TR0 channel as used in one embodiment of the presentinvention. Seven-bit register 1601 with stages Q1 through Q7 uses thesignal TXPNCLK to clock in TR0 data when the TR0EN signal is asserted.The output value of stages Q1, Q2, Q3, Q4, Q6, and Q7 are each combinedusing EXCLUSIVE-OR Logic 1602, 1603 to produce respective I and Qchannel FEC data for the TR0 channel FECTR0DI and FECTR0DQ.

Two output symbol streams FECTR0DI and FECTR0DQ are generated. TheFECTR0DI symbol stream is generated by EXCLUSIVE-OR logic 1602 of shiftregister outputs corresponding to bits 6, 5, 4, 3, and 0, (Octal 171)and is designed as In phase component “I” of the transmit messagechannel data. The symbol stream FECTR0DQ is likewise generated byEXCLUSIVE-OR logic 1603 of shift register outputs from bits 6, 4, 3, 1and 0, (Octal 133) and is designated as Quadrature component “Q” of thetransmit message channel data. Two symbols are transmitted to representa single encoded bit creating the redundancy necessary to enable errorcorrection to take place on the receiving end.

Referring to FIG. 14, the shift enable clock signal for the transmitmessage channel data is generated by the control timing logic 1401. Theconvolutionally encoded transmit message channel output data for eachchannel is applied to the respective spreader 1420, 1421, 1422, 1423,1424 which multiplies the transmit message channel data by itspreassigned spreading code from code generator 1402. This spreading codeis generated by control 1303 as previously described, and is called arandom pseudonoise signature code (PN-code).

The output signal of each spreader 1420, 1421, 1422, 1423, 1424 is aspread transmit data channel. The operation of the spreader is asfollows: the spreading of channel output (I+jQ) multiplied by a randomsequence (PNI+jPNQ) yields the in-phase component I of the result beingcomposed of (I xor PNI) and (−Q xor PNQ). Quadrature component Q of theresult is (Q xor PNI) and (I xor PNQ). Since there is no channel datainput to the pilot channel logic (I=1, Q values are prohibited), thespread output signal for pilot channels yields the respective sequencesPNI for 1 component and PNQ for Q component.

The combiner 1430 receives the I and Q spread transmit data channels andcombines the channels into an I modem transmit data signal (TXIDAT) anda Q modem transmit data signal (TXQDAT). The I-spread transmit data andthe Q-spread transmit data are added separately.

For an SU, the CDMA modem transmit section 1301 includes the FIR filtersto receive the I and Q channels from the combiner to provide pulseshaping, close-in spectral control and x/sin (x) correction for thetransmitted signal. Separate but identical FIR filters receive the I andQ spread transmit data streams at the chipping rate, and the outputsignal of each of the filters is at twice the chipping rate. Theexemplary FIR filters are 28 tap even symmetrical filters, whichupsample (interpolate) by 2. The upsampling occurs before the filtering,so that 28 taps refers to 28 taps at twice the chipping rate, and theupsampling is accomplished by setting every other sample a zero.Exemplary coefficients are shown in Table 11.

TABLE 11 Coefficient Values Coeff. No.: 0 1 2 3 4 5 6 7 8 9 10 11 12 13Value: 3 −11 −34 −22 19 17 −32 −19 52 24 −94 −31 277 468 Coeff. No. 1415 16 17 18 19 20 21 22 24 25 26 27 Value 277 −31 −94 24 52 −19 −32 1719 −22 −34 −11 3

XXIV. CDMA Modem Receiver Section

Referring to FIGS. 9 and 12, the RF receiver 950 of the presentembodiment accepts analog input I and Q CDMA channels, which aretransmitted to the CDMA modems 1210, 1211, 1212, 1215 through the MIUs931, 932, 933 from the VDC 940. These I and Q CMDA channel signals aresampled by the CDMA modem receive section 1302 (shown in FIG. 13) andconverted to I and Q digital receive message signals using an analog todigital (A/D) converter 1730, shown in FIG. 17. The sampling rate of theA/D converter of the exemplary embodiment of the present invention isequivalent to the despreading code rate. The I and Q digital receivemessage signals are then despread with correlators using six differentcomplex spreading code sequences corresponding to the despreading codesequences of the four channels (TR0, TR1, TR2, TR3), APC information andthe pilot code.

Time synchronization of the receiver to the received signal is separatedinto two phases; there is an initial acquisition phase and then atracking phase after the signal timing has been acquired. The initialacquisition is done by shifting the phase of the locally generated pilotcode sequence relative to the received signal and comparing the outputof the pilot despreader to a threshold. The method used is calledsequential search. Two thresholds (match and dismiss) are calculatedfrom the auxiliary despreader. Once the signal is acquired, the searchprocess is stopped and the tracking process begins. The tracking processmaintains the code generator 1304 (shown in FIGS. 13 and 17) used by thereceiver in synchronization with the incoming signal. The tracking loopused is the delay-locked loop (DLL) and is implemented in theacquisition & track 1701 and the IPM 1702 blocks of FIG. 17.

In FIG. 13, the modem controller 1303 implements the phase lock loop(PLL) as a software algorithm in SW PLL logic 1724 of FIG. 17 thatcalculates the phase and frequency shift in the received signal relativeto the transmitted signal. The calculated phase shifts are used toderotate the phase shifts in rotate and combine blocks 1718, 1719, 1720,1721 of the multipath data signals for combining to produce outputsignals corresponding to receive channels TR0′, TR1′, TR2′, TR3′. Thedata is then Viterbi decoded in Viterbi decoders 1713, 1714, 1715, 1716to remove the convolutional encoding in each of the received messagechannels.

FIG. 17 indicates that the Code Generator 1304 provides the codesequences Pn_(i)(t), i=1, 2, . . . I used by the receive channeldespreaders 1703, 1704, 1705, 1706, 1707, 1708, 1709. The code sequencesgenerated are timed in response to the SYNK signal of the system clocksignal and are determined by the CCNTRL signal from the modem controller1303 shown in FIG. 13. Referring to FIG. 17, the CDMA modem receiversection 1302 includes adaptive matched filter (AMF) 1710, channeldespreaders 1703, 1704, 1705, 1706, 1707, 1708, 1709, pilot AVC 1711,auxiliary AVC 1712, Viterbi decoders 1713, 1714, 1715, 1716, modemoutput interface (MOI) 1717, rotate and combine logic 1718, 1719, 1720,1721, AMF weight generator 1722, and quantile estimation logic 1723 and1733.

In another embodiment of the invention, the CDMA modem receiver alsoincludes a bit error integrator to measure the BER of the channel andidle code insertion logic between the Viterbi decoders 1713, 1714, 1715,1716 and the MOI 1717 to insert idle codes in the event of loss of themessage data.

The AMF 1710 resolves multipath interference introduced by the airchannel. The exemplary AMF 1710 uses an 11 stage complex FIR filter asshown in FIG. 18. The received I and Q digital message signals arereceived at the register 1820 from the A/D 1730 of FIG. 17 and aremultiplied in multipliers 1801, 1802, 1803, 1810, 1811 by I and Qchannel weights W1 to W11 received from AMF weight generator 1722 ofFIG. 17. In the exemplary embodiment, the A/D 1730 provides the I and Qdigital receive message signal data as 2's complement values, 6 bits for1 and 6 bits for Q which are clocked through an 11 stage shift register1820 responsive to the receive spreading-code clock signal RXPNCLK. Thesignal RXPNCLK is generated by the timing section 1401 of codegeneration logic 1304. Each stage of the shift register is tapped andcomplex multiplied in the multipliers 1801, 1802, 1803, 1810, 1811 byindividual (6-bit I and 6-bit Q) weight values to provide 11tap-weighted products which are summed in adder 1830, and limited to7-bit I and 7-bit Q values.

The CDMA modem receive section 1302 (shown in FIG. 13) providesindependent channel despreaders 1703, 1704, 1705, 1706, 1707, 1708, 1709(shown in FIG. 17) for despreading the message channels. The describedembodiment despreads 7 message channels, each despreader accepting a1-bit I by 1-bit Q despreading code signal to perform a complexcorrelation of this code against a 8-bit I by 8-bit Q data input. The 7despreaders correspond to the 7 channels: traffic channel 0 (TR0′),TR1′, TR2′, TR3′, AUX (a spare channel), APC and pilot (PLT).

The pilot AVC 1711 shown in FIG. 19 receives the I and Q pilot spreadingcode sequence values PCI and PCQ into shift register 1920 responsive tothe timing signal RXPNCLK, and includes 11 individual despreaders 1901through 1911 each correlating the I and Q digital receive message signaldata with a one chip delayed version of the same pilot code sequence.Signals OE1, OE2, . . . OE11 are used by the modem control 1303 toenable the despreading operation. The output signals of the despreadersare combined in combiner 1920 forming correlation signal DSPRDAT of thePilot AVC 1711, which is received by the ACQ & track logic 1701 (shownin FIG. 17), and ultimately by modem controller 1303 (shown in FIG. 13).The ACQ & track logic 1701 uses the correlation signal value todetermine if the local receiver is synchronized with its remotetransmitter.

The auxiliary AVC 1712 also receives the I and Q digital receive messagesignal data and, in the described embodiment, includes four separatedespreaders 2001, 2002, 2003, 2004 as shown in FIG. 20. Each despreaderreceives and correlates the I and Q digital receive message data withdelayed versions of the same despreading code sequence PARI and PARQwhich are provided by code generator 1304 input to and contained inshift register 2020. The output signals of the despreaders 2001, 2002,2003, 2004 are combined in combiner 2030 which provides noisecorrelation signal ARDSPRDAT. The auxiliary AVC spreading code sequencedoes not correspond to any transmit spreading code sequence of thesystem. Signals OE1, OE2, . . . OE4 are used by the modem control 1303to enable the despreading operation. The Auxiliary AVC 1712 provides anoise correlation signal ARDSPRDAT from which quantile estimates arecalculated by the quantile estimator 1733, and provides a noise levelmeasurement to the ACQ & Track logic 1701 (shown in FIG. 17) and modemcontroller 1303 (shown in FIG. 13).

Each despread channel output signal corresponding to the receivedmessage channels TR0′, TR1′, TR2′, and TR3′ is input to a correspondingViterbi decoder 1713, 1714, 1715, 1716 shown in FIG. 17 which performsforward error correction on convolutionally encoded data. The Viterbidecoders of the exemplary embodiment have a constraint length of K=7 anda rate of R=½. The decoded despread message channel signals aretransferred from the CDMA modem to the PCM Highway 1201 through the MOI1717. The operation of the MOI is essentially the same as the operationof the MISR of the transmit section 1301 (shown in FIG. 13) except inreverse.

The CDMA modem receiver section 1302 implements several differentalgorithms during different phases of the acquisition, tracking anddespreading of the receive CDMA message signal.

When the received signal is momentarily lost (or severely degraded) theidle code insertion algorithm inserts idle codes in place of the lost ordegraded receive message data to prevent the user from hearing loudnoise bursts on a voice call. The idle codes are sent to the MOI 1717(shown in FIG. 17) in place of the decoded message channel output signalfrom the Viterbi decoders 1713, 1714, 1715, 1716. The idle code used foreach traffic channel is programmed by the Modem Controller 1303 bywriting the appropriate pattern IDLE to the MOI, which in the presentembodiment is a 8 bit word for a 64 kbs stream, 4 bit word for a 32 kbsstream.

XXV. Modem Algorithms for Acquisition and Tracking of Received PilotSignal

The acquisition and tracking algorithms are used by the receiver todetermine the approximate code phase of a received signal, synchronizethe local modem receiver despreaders to the incoming pilot signal, andtrack the phase of the locally generated pilot code sequence with thereceived pilot code sequence. Referring to FIGS. 13 and 17, thealgorithms are performed by the modem controller 1303, which providesclock adjust signals to code generator 1304. These adjust signals causethe code generator for the despreaders to adjust locally generated codesequences in response to measured output values of the pilot rake 1711and quantile values from quantile estimation logic 1733. Quantile valuesare noise statistics measured from the in-phase and quadrature channelsfrom the output values of the AUX vector correlator 1712 (shown in FIG.17). Synchronization of the receiver to the received signal is separatedinto two phases; an initial acquisition phase and a tracking phase. Theinitial acquisition phase is accomplished by clocking the locallygenerated pilot spreading code sequence at a higher or lower rate thanthe received signal's spreading code rate, sliding the locally generatedpilot spreading code sequence and performing sequential probabilityratio test (SPRT) on the output of the pilot vector correlator 1711. Thetracking phase maintains the locally generated spreading code pilotsequence in synchronization with the incoming pilot signal. Details ofthe quantile estimation logic 1723 and 1733 may be found in U.S. Pat.No. 5,535,238 entitled “ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUMCOMMUNICATIONS SYSTEM” which is incorporated by reference herein for itsteachings on APC systems.

The SU cold acquisition algorithm is used by the SU CDMA modem when itis first powered up, and therefore has no knowledge of the correct pilotspreading code phase, or when an SU attempts to reacquiresynchronization with the incoming pilot signal but has taken anexcessive amount of time. The cold acquisition algorithm is divided intotwo sub-phases. The first subphase consists of a search over the length233415 code used by the FBCH. Once this sub-code phase is acquired, thepilot's 233415×128 length code is known to within an ambiguity of 128possible phases. The second subphase is a search of these remaining 128possible phases. In order not to lose synch with the FBCH, in the secondphase of the search, it is desirable to switch back and forth betweentracking of the FBCH code and attempting acquisition of the pilot code.

The RCS acquisition of short access pilot (SAXPT) algorithm is used byan RCS CDMA modem to acquire the SAXPT pilot signal of an SU. Additionaldetails of this technique are described in Section XXXXII hereinafterentitled “A Method Of Controlling Initial Power Ramp-Up In CDMA SystemsBy Using Short Codes” filed on even date herewith and hereinincorporated by reference. The algorithm is a fast search algorithmbecause the SAXPT is a short code sequence of length N, whereN=chips/symbol, and ranges from 45 to 195, depending on the system'sbandwidth. The search cycles through all possible phases untilacquisition is complete.

The RCS acquisition of the long access pilot (LAXPT) algorithm beginsimmediately after acquisition of SAXPT. The SU's code phase is knownwithin a multiple of a symbol duration, so in the exemplary embodimentof the invention there may be 7 to 66 phases to search within the roundtrip delay from the RCS. This boundary is a result of the SU pilotsignal being synchronized to the RCS global pilot signal.

The re-acquisition algorithm begins when loss of lock (LOL) occurs. AZ-search algorithm is used to speed the process on the assumption thatthe code phase has not drifted far from where it was the last time thesystem was locked. The RCS uses a maximum width of the Z-search windowsbounded by the maximum round trip propagation delay.

The pre-track period immediately follows the acquisition orre-acquisition algorithms and immediately precedes the trackingalgorithm. Pre-track is a fixed duration period during which the receivedata provided by the modem is not considered valid. The pre-track periodallows other modem algorithms, such as those used by the SW PLL 1724,acquisition and tracking circuit 1701, AMF weight generator 1722, toprepare and adapt to the current channel. The pre-track period is twoparts. The first part is the delay while the code tracking loop pullsin. The second part is the delay while the AMF tap weight calculationsare performed by the AMF weight generator 1722 to produce settledweighting coefficients. Also in the second part of the pre-track period,the carrier tracking loop is allowed to pull in by the SW PLL 1724, andthe scalar quantile estimates are performed in the quantile estimationlogic 1723.

The tracking process is entered after the pre-track period ends. Thisprocess is actually a repetitive cycle and is the only process phaseduring which receive data provided by the modem may be considered valid.The following operations are performed during this phase: AMF tap weightupdate, carrier tracking, code tracking, vector quantile update, scalarquantile update, code lock check, derotation and symbol summing andpower control (forward and reverse)

If LOL is detected, the modem receiver terminates the track algorithmand automatically enters the reacquisition algorithm. In the SU, a LOLcauses the transmitter to be shut down. In the RCS, LOL causes forwardpower control to be disabled with the transmit power held constant atthe level immediately prior to loss of lock. It also causes the returnpower control information being transmitted to assume a “010101 . . . ”pattern, causing the SU to hold its transmit power constant. This can beperformed using the signal lock check function which generates the resetsignal to the acquisition and tracking circuit 1701.

Two sets of quantile statistics are maintained, one by quantileestimation logic 1733 and the other by the scalar quantile estimator1723. Both are used by the modem controller 1303. The first set is the“vector” quantile information, so named because it is calculated fromthe vector of four complex values generated by the AUX AVC receiver1712. The second set is the scalar quantile information, which iscalculated from the single complex value AUX signal that is output fromthe AUX despreader 1707. The two sets of information represent differentsets of noise statistics used to maintain a pre-determined probabilityof false alarm (Pfa). The vector quantile data is used by theacquisition and reacquisition algorithms implemented by the modemcontroller 1303 to determine the presence of a received signal in noise,and the scalar quantile information is used by the code lock checkalgorithm.

For both the vector and scalar cases, quantile information consists ofcalculated values of lambda0 through lambda2, which are boundary valuesused to estimate the probability distribution function (p.d.f.) of thedespread receive signal and determine whether the modem is locked to thePN code. The aux_power value used in the following C-subroutine is themagnitude squared of the AUX signal output of the scalar correlatorarray for the scalar quantiles, and the sum of the magnitudes squaredfor the vector case. In both cases the quantiles are then calculatedusing the following C-subroutine:for (n=0; n<3; n++){lambda[n]+=(lambda[n]<Aux_Power)?CG[n]: GM[n]; }where CG[n] are positive constants and GM[n] are negative constants,(different values are used for scalar and vector quantiles).

During the acquisition phase, the search of the incoming pilot signalwith the locally generated pilot code sequence employs a series ofsequential tests to determine if the locally generated pilot code hasthe correct code phase relative to the received signal. The searchalgorithms use the sequential probability ratio test (SPRT) to determinewhether the received and locally generated code sequences are in phase.The speed of acquisition is increased by parallelism resulting fromhaving a multi-fingered receiver. For example, in the describedembodiment of the invention the main pilot rake 1711 has a total of 11fingers representing a total phase period of 11 chip periods. Foracquisition 8 separate SPRTs are implemented, with each SPRT observing a4 chip window. Each window is offset from the previous window by onechip, and in a search sequence any given code phase is covered by 4windows. If all 8 of the SPRT tests are rejected, then the set ofwindows is moved by 8 chips. If any of the SPRT's is accepted, then thecode phase of the locally generated pilot code sequence is adjusted toattempt to center the accepted SPRT's phase within the pilot AVC. It islikely that more than one SPRT reaches the acceptance threshold at thesame time. A table lookup is used to cover all 256 possible combinationsof accept/reject and the modem controller uses the information toestimate the correct center code phase within the pilot rake 1711. EachSPRT is implemented as follows (all operations occur at 64 k symbolrate): Denote the fingers' output level values as I_Finger[n] andQ_Finger[n], where n=0 . . . 10 (inclusive, 0 is earliest (mostadvanced) finger), then the power of each window is:

${{Power}\mspace{14mu}{{Window}\lbrack i\rbrack}} = {\sum\limits_{n = 0}^{10}\;( {{{{I\_}{Finger}}^{2}\lbrack n\rbrack} + {{{Q\_}{Finger}}^{2}\lbrack n\rbrack}} )}$To implement the SPRT's the modem controller then performs for each ofthe windows the following calculations which are expressed as apseudo-code subroutine:

/* find bin for Power */ tmp = SIGMA[0]; for (k = 0; k< 3; k++) {  if(Power > lambda [k]) tmp = SIGMA[k+1]; } test_statistic += tmp; /*update statistic */ if(test_statistic > ACCEPTANCE_THRESHOLD)you've gotACQ; else if (test_statistic < DISMISSAL_THRESHOLD) {  forget this codephase; } else keep trying - get more statistics;where lambda [k] are as defined in the above section on quantileestimation, and SIGMA[k], ACCEPTANCE_THRESHOLD and DISMISSAL_THRESHOLDare predetermined constants. Note that SIGMA[k] is negative for lowvalues of k, and positive for right values of k, such that theacceptance and dismissal thresholds can be constants rather than afunction of how many symbols worth of data have been accumulated in thestatistic.

The modem controller determines which bin delimited by the values oflambda[k] the power level falls into which allows the modem controllerto develop an approximate statistic.

For the present algorithm, the control voltage is formed as ε=y^(T) By,where y is a vector formed from the complex valued output values of thepilot vector correlator 1711, and B is a matrix consisting of theconstant values pre-determined to maximize the operating characteristicswhile minimizing the noise as described previously with reference to thequadratic detector.

To understand the operation of the quadratic detector, it is useful toconsider the following. A spread spectrum signal, s(t) is passed througha multipath channel with an impulse response h_(c)(t). The basebandspread signal is described by Equation (30):

$\begin{matrix}{{s(t)} = {\sum\limits_{i}{C_{i}{p( {t - {i\; T_{c}}} )}}}} & {{Equation}\mspace{14mu}(30)}\end{matrix}$where C_(i) is a complex spreading code symbol, p(t) is a predefinedchip pulse and T_(c) is the chip time spacing, where T_(c)=1/R_(c) andR_(c) is the chip rate.

The received baseband signal is represented by Equation (31):

$\begin{matrix}{{r(t)} = {{\sum\limits_{i}{C_{i}{q( {t - {i\; T_{c}} - \tau} )}}} + {n(t)}}} & {{Equation}\mspace{14mu}(31)}\end{matrix}$where q(t)=p(t)*h_(c)(t), t is an unknown delay and n(t) is additivenoise. The received signal is processed by a filter, h_(R)(t), so thewaveform, x(t), to be processed is given by Equation (32):

$\begin{matrix}{{x(t)} = {{\sum\limits_{i}{C_{i}{f( {t - {i\; T_{c}} - \tau} )}}} + {z(t)}}} & {{Equation}\mspace{14mu}(32)}\end{matrix}$where f(t)=q(t)*h_(R)(t) and z(t)=n(t)*h_(R)(t).

In the exemplary receiver, samples of the received signal are taken atthe chip rate, that is to say, 1/T_(c). These samples, x(mT_(c)+τ′), areprocessed by an array of correlators that compute, during the r^(th)correlation period, the quantities given by Equation (33):

$\begin{matrix}{v_{k}^{(r)} = {\sum\limits_{m = {r\; L}}^{{r\; L} + L - 1}{{x( {{m\; T_{c}} + \tau^{\prime}} )}C_{m + k}^{*}}}} & {{Equation}\mspace{14mu}(33)}\end{matrix}$These quantities are composed of a noise component w_(k) ^((r)) and adeterministic component y_(k) ^((r)) given by Equation (34):

$\begin{matrix}{y_{k}^{(r)} = {{E\lbrack v_{k}^{(r)} \rbrack} = {L\;{f( {{k\; T_{c}} + \tau^{\prime} - \tau} )}}}} & {{Equation}\mspace{14mu}(34)}\end{matrix}$In the sequel, the time index r may be suppressed for ease of writing,although it is to be noted that the function f(t) changes slowly withtime.

The samples are processed to adjust the sampling phase, τ′, in anoptimum fashion for further processing by the receiver, such as matchedfiltering. This adjustment is described below. To simplify therepresentation of the process, it is helpful to describe it in terms ofthe function f(t−τ), where the time-shift, τ, is to be adjusted. It isnoted that the function f(t+τ) is measured in the presence of noise.Thus, it may be problematical to adjust the phase τ′ based onmeasurements of the signal f(t+τ). To account for the noise, thefunction v(τ): v(t)=f(t)+m(t) is introduced, where the term m(t)represents a noise process. The system processor may be derived based onconsiderations of the function v(t).

The process is non-coherent and therefore is based on the envelope powerfunction |v(t+τ)|². The functional e(τ′) given in Equation (35) ishelpful for describing the process:

$\begin{matrix}{{e( \tau^{\prime} )} = {{\int_{- \infty}^{0}{{{v( {t + \tau^{\prime} - \tau} )}}^{2}{\mathbb{d}t}}} - {\int_{0}^{\infty}{{{v( {t + \tau^{\prime} - \tau} )}}^{2}{\mathbb{d}t}}}}} & {{Equation}\mspace{14mu}(35)}\end{matrix}$The shift parameter is adjusted for e(τ′)=0, which occurs when theenergy on the interval (−∞, τ′−τ] equals that on the interval [τ′−τ, ∞).The error characteristic is monotonic and therefore has a single zerocrossing point. This is the desirable quality of the functional. Adisadvantage of the functional is that it is ill-defined because theintegrals are unbounded when noise is present. Nevertheless, thefunctional e(τ′) may be cast in the form given by Equation (36):

$\begin{matrix}{{e( \tau^{\prime} )} = {\int_{- \infty}^{\infty}{{w(t)}{{v( {t + \tau^{\prime} - \tau} )}}^{2}{\mathbb{d}t}}}} & {{Equation}\mspace{14mu}(36)}\end{matrix}$where the characteristic function w(t) is equal to sgn(t), the signumfunction.

To optimize the characteristic function w(t), it is helpful to define afigure of merit, F, as set forth in Equation (37):

$\begin{matrix}{F = \frac{\lbrack \overset{\_}{{e( {\tau_{0}^{\prime} + T_{A}} )} - {e( {\tau_{0}^{\prime} - T_{A}} )}} \rbrack^{2}}{{VAR}\{ {e( \tau_{0}^{\prime} )} \}}} & {{Equation}\mspace{14mu}(37)}\end{matrix}$The numerator of F is the numerical slope of the mean errorcharacteristic on the interval [−T_(A),T_(A)] surrounding the trackedvalue, τ₀′. The statistical mean is taken with respect to the noise aswell as the random channel, h_(c)(t). It is desirable to specify astatistical characteristic of the channel in order to perform thisstatistical average. For example, the channel may be modeled as a widesense stationary uncorrelated scattering (WSSUS) channel with impulseresponse h_(c)(t) and a white noise process U(t) that has an intensityfunction g(t) as shown in Equation (38):h _(c)(t)=√{square root over (g(t))}U(t)  Equation (38)The variance of e(τ) is computed as the mean square value of thefluctuation:e′(τ)=e(τ)−<e(τ)>  Equation (39)where <e(τ)> is the average of e(τ) with respect to the noise.

Optimization of the figure of merit F with respect to the function w(t)may be carried out using well-known variational methods of optimization.Once the optimal w(t) is determined, the resulting processor may beapproximated accurately by a quadratic sample processor which is derivedas follows. By the sampling theorem, the signal v(t), bandlimited to abandwidth W may be expressed in terms of its samples as shown inEquation (40):v(t)=Σv(k/W)sin c[(Wt−k)π]  Equation (40)substituting this expansion into Equation (36) results in an infinitequadratic form in the samples v(k/W+τ′−τ). Making the assumption thatthe signal bandwidth equals the chip rate allows the use of a samplingscheme that is clocked by the chip clock signal to be used to obtain thesamples. These samples, v_(k) are represented by Equation (41):v _(k) =v(kT _(c)+τ′−τ)  Equation (41)This assumption leads to a simplification of the implementation. It isvalid if the aliasing error is small.

In practice, the quadratic form that is derived is truncated. An examplenormalized B matrix is given below in Table 12. For this example, anexponential delay spread profile g(t)=exp(−t/τ) is assumed with 1 equalto one chip. An aperture parameter T_(A) equal to one and one-half chipshas also been assumed. The underlying chip pulse has a raised cosinespectrum with a 20% excess bandwidth.

TABLE 12 Example B Matrix 0 0 0 0 0 0 0 0 0 0 0 0 0 −0.1 0 0 0 0 0 0 0 00 −0.1 0.22 0.19 −0.19 0 0 0 0 0 0 0 0 0.19 1 0.45 −0.2 0 0 0 0 0 0 0−0.19 0.45 0.99 0.23 0 0 0 0 0 0 0 0 −0.2 0.23 0 −0.18 0.17 0 0 0 0 0 00 0 −0.18 −0.87 −0.42 0.18 0 0 0 0 0 0 0 0.17 −0.42 −0.92 −0.16 0 0 0 00 0 0 0 0.18 −0.16 −0.31 0 0 0 0 0 0 0 0 0 0 0 −0.13 0 0 0 0 0 0 0 0 0 00 0Code tracking is implemented via a loop phase detector that isimplemented as follows. The vector y is defined as a column vector whichrepresents the 11 complex output level values of the pilot AVC 1711, andB denotes an 11×11 symmetric real valued coefficient matrix withpre-determined values to optimize performance with the non-coherentpilot AVC output values y. The output signal e of the phase detector isgiven by Equation (42):e=y^(T)By  Equation (42)The following calculations are then performed to implement aproportional plus integral loop filter and the VCO:x[n]=x[n−1]+bez[n]=z[n−1]+x[n]+aefor b and a which are constants chosen from modeling the system tooptimize system performance for the particular transmission channel andapplication, and where x[n] is the loop filter's integrator output valueand z [n] is the VCO output value. The code phase adjustments are madeby the modem controller in the following C-subroutine:

if (z > zmx) { delay phase 1/16 chip; z −= zmax; } else if(z < −zmax) {advance phase 1/16 chip; z += zmax; }

A different delay phase could be used in the above pseudo-codeconsistant with the present invention.

The AMF tap-weight update algorithm of the AMF weight generator 1722occurs periodically to de-rotate and scale the phase of each fingervalue of the pilot rake 1711 by performing a complex multiplication ofthe pilot AVC finger value with the complex conjugate of the currentoutput value of the carrier tracking loop and applying the product to alow pass filter and form the complex conjugate of the filter values toproduce AMF tap-weight values, which are periodically written into theAMF filters of the CDMA modem.

The lock check algorithm, shown in FIG. 17, is implemented by the modemcontroller 1303 performing SPRT operations on the output signal of thescalar correlator array. The SPRT technique is the same as that for theacquisition algorithms, except that the acceptance and rejectionthresholds are changed to increase the probability of detection of lock.

Carrier tracking is accomplished via a second order loop that operateson the pilot output values of the scalar correlated array. The phasedetector output is the hard limited version of the quadrature componentof the product of the (complex valued) pilot output signal of the scalarcorrelated array and the VCO output signal. The loop filter is aproportional plus integral design. The VCO is a pure summation,accumulated phase error φ, which is converted to the complex phaser cosφ+j sin φ using a look-up table in memory.

The previous description of acquisition and tracking algorithm focuseson a non-coherent method because the acquisition and tracking algorithmdescribed requires non-coherent acquisition followed by non-coherenttracking because during acquisition a coherent reference is notavailable until the AMF, pilot AVC, aux AVC, and DPLL are in anequilibrium state. However, it is known in the art that coherenttracking and combining is always optimal because in non-coherenttracking and combining the output phase information of each pilot AVCfinger is lost. Consequently, another embodiment of the inventionemploys a two step acquisition and tracking system, in which thepreviously described non-coherent acquisition and tracking algorithm isimplemented first, and then the algorithm switches to a coherenttracking method. The coherent combining and tracking method is similarto that described previously, except that the error signal tracked is ofthe form:ε=y^(T)Ay  Equation (43)where y is defined as a column vector which represents the 11 complexoutput level values of the pilot AVC 1711, and A denotes an 11×11symmetric real valued coefficient matrix with pre-determined values tooptimize performance with the coherent pilot AVC outputs y. An exemplaryA matrix is shown below.

$\begin{matrix}{A = \begin{matrix}1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & {- 1} & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & {- 1} & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & {- 1} & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & {- 1} & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & {- 1}\end{matrix}} & {{Equation}\mspace{11mu}(44)}\end{matrix}$

Referring to FIG. 9, the video distribution controller board (VDC) 940of the RCS is connected to each MIU 931, 932, 933 and the RFtransmitters/receivers 950. The VDC 940 is shown in FIG. 21. The datacombiner circuitry (DCC) 2150 includes a data demultiplexer 2101, datasummer 2102, FIR filters 2103, 2104 and a driver 2111. The DCC 2150: 1)receives the weighted CDMA modem I and Q data signal MDAT from each ofthe MIUs, 931, 932, 933, 2) sums the I and Q data with the digitalbearer channel data from each MIU 931, 932, 933, 3) and sums the resultwith the broadcast data message signal BCAST and the global pilotspreading code GPILOT provided by the master MIU modem 1210, 4) bandshapes the summed signals for transmission, and 5) produces analog datasignal for transmission to the RF transmitter/receiver.

FIR filters 2103, 2104 are used to modify the MIU CDMA transmit I and Qmodem data before transmission. The WAC transfers FIR filter coefficientdata through the serial port link 912 through the VDC controller 2120and to the FIR filters 2103, 2104. Each FIR filter 2103, 2104 isconfigured separately. The FIR Filters 2103, 2104 employ upsampling tooperate at twice the chip rate so zero data values are sent after everyMIU CDMA transmit modem DATI and DATQ value to produce FTXI and FTXQ.

The VDC 940 distributes the AGC signal AGCDATA from the AGC 1750 of theMIUs 931, 932, 933 to the RF transmitter/receiver 950 through thedistribution interface (DI) 2110. The VDC DI 2110 receives data RXI andRXQ from the RF transmitter/receiver and distributes the signal asVDATAI and VDATAQ to MIUs 931, 932, 933.

Referring to FIG. 21, the VDC 940 also includes a VDC controller 2120which monitors status and fault information signals MIUSTAT from MIUsand connects to the serial link 912 and HSBS 970 to communicate with WAC920 shown in FIG. 9. The VDC controller 2120 includes a microprocessor,such as an Intel 8032 microcontroller, an oscillator (not shown)providing timing signals, and memory (not shown). The VDC controllermemory includes a flash PROM (not shown) to contain the controllerprogram code for the 8032 microprocessor, and an SRAM (not shown) tocontain the temporary data written to and read from memory by themicroprocessor.

Referring to FIG. 9, the present invention includes a RFtransmitter/receiver 950 and power amplifier section 960. Referring toFIG. 22, the RF transmitter/receiver 950 is divided into three sections:the transmitter module 2201, the receiver module 2202, and the frequencysynthesizer 2203. Frequency synthesizer 2203 produces a transmit carrierfrequency TFREQ and a receive carrier frequency RFREQ in response to afrequency control signal FREQCTRL received from the WAC 920 on theserial link 912. In the transmitter module 2201, the input analog I andQ data signals TXI and TXQ from the VDC are applied to the quadraturemodulator 2220, which also receives a transmit carrier frequency signalTFREQ from the frequency synthesizer 2203 to produce a quadraturemodulated transmit carrier signal TX. The analog transmit carriermodulated signal, an upconverted RF signal, TX is then applied to thetransmit power amplifier 2252 of the power amplifier 960. The amplifiedtransmit carrier signal is then passed through the high power passivecomponents (HPPC) 2253 to the Antenna 2250, which transmits theupconverted RF signal to the communication channel as a CDMA RF signal.In one embodiment of the invention, the transmit power amplifier 2252comprises eight amplifiers of approximately 60 watts peak-to-peak each.

The HPPC 2253 comprises a lightning protector, an output filter, a 10 dBdirectional coupler, an isolator, and a high power termination attachedto the isolator.

A receive CDMA RF signal is received at the antenna 2250 from the RFchannel and passed through the HPPC 2253 to the receive power amplifier2251. The receive power amplifier 2251 includes, for example, a 30 wattpower transistor driven by a 5 watt transistor. The RF receive module2202 has quadrature modulated receive carrier signal RX from the receivepower amplifier. The receive module 2202 includes a quadraturedemodulator 2210 which takes the receive carrier modulated signal RX andthe receive carrier frequency signal RFREQ from the frequencysynthesizer 2203, synchronously demodulates the carrier and providesanalog I and Q channels. These channels are filtered to produce thesignals RXI and RXQ, which are transferred to the VDC 940.

XXVI. The Subscriber Unit

FIG. 23 shows the subscriber unit (SU) of one embodiment of the presentinvention. As shown, the SU includes an RF section 2301 including a RFmodulator 2302, RF demodulator 2303 and splitter/isolator 2304 whichreceive global and assigned logical channels including traffic andcontrol messages and global pilot signals in the forward link CDMA RFchannel signal, and transmit assigned channels and reverse pilot signalsin the reverse link CDMA RF channel. The forward and reverse links arereceived and transmitted respectively through antenna 2305. The RFsection employs, in one exemplary embodiment, a conventional dualconversion superheterodyne receiver having a synchronous demodulatorresponsive to the signal ROSC. Selectivity of such a receiver isprovided by a 70 MHz transversal SAW filter (not shown). The RFmodulator includes a synchronous modulator (not shown) responsive to thecarrier signal TOSC to produce a quadrature modulated carrier signal.This signal is stepped up in frequency by an offset mixing circuit (notshown).

The SU further includes a subscriber line interface 2310, including thefunctionality of a control (CC) generator, a data interface 2320, anADPCM encoder 2321, an ADPCM decoder 2322, an SU controller 2330, an SUclock signal generator 2331, memory 2332 and a CDMA modem 2340, which isessentially the same as the CDMA modem 1210 described above withreference to FIG. 13. It is noted that data interface 2320, ADPCMencoder 2321 and ADPCM decoder 2322 are typically provided as a standardADPCM encoder/decoder chip.

The forward link CDMA RF channel signal is applied to the RF demodulator2303 to produce the forward link CDMA signal. The forward link CDMAsignal is provided to the CDMA modem 2340, which acquiressynchronization with the global pilot signal, produces global pilotsynchronization signal to the clock 2331, to generate the system timingsignals, and despreads the plurality of logical channels. The CDMA modem2340 also acquires the traffic messages RMESS and control messages RCTRLand provides the traffic message signals RMESS to the data interface2320 and receive control message signals RCTRL to the SU controller2330.

The receive control message signals RCTRL include a subscriberidentification signal, a coding signal and bearer modification signals.The RCTRL may also include control and other telecommunication signalinginformation. The receive control message signal RCTRL is applied to theSU controller 2330, which verifies that the call is for the SU from thesubscriber identification value derived from RCTRL. The SU controller2330 determines the type of user information contained in the trafficmessage signal from the coding signal and bearer rate modificationsignal. If the coding signal indicates the traffic message is ADPCMcoded, the traffic message RVMESS is sent to the ADPCM decoder 2322 bysending a select message to the data interface 2320. The SU controller2330 outputs an ADPCM coding signal and bearer rate signal derived fromthe coding signal to the ADPCM decoder 2322. The traffic message signalRVMESS is the input signal to the ADPCM decoder 2322, where the trafficmessage signal is converted to a digital information signal RINF inresponse to the values of the input ADPCM coding signal.

If the SU controller 2330 determines the type of user informationcontained in the traffic message signal from the coding signal is notADPCM coded, then RDMESS passes through the ADPCM encoder transparently.The traffic message RDMESS is transferred from the data interface 2320directly to the interface controller (IC) 2312 of the subscriber lineinterface 2310.

The digital information signal RINF or RDMESS is applied to thesubscriber line interface 2310, including an interface controller (IC)2312 and line interface (LI) 2313. For the exemplary embodiment the ICis an extended PCM interface controller (EPIC) and the LI is asubscriber line interface circuit (SLIC) for POTS which corresponds toRINF type signals and an ISDN Interface for ISDN which corresponds toRDMESS type signals. The EPIC and SLIC circuits are well known in theart. The subscriber line interface 2310 converts the digital informationsignal RINF or RDMESS to the user defined format. The user definedformat is provided to the IC 2312 from the SU Controller 2330. The LI2310 includes circuits for performing such functions as A-law or μ-lawconversion, generating dial tone and generating or interpretingsignaling bits. The line interface also produces the user informationsignal to the SU user 2350 as defined by the subscriber line interface,for example POTS voice, voiceband data or ISDN data service.

For a reverse link CDMA RF channel, a user information signal is appliedto the LI 2313 of the subscriber line interface 2310, which outputs aservice type signal and an information type signal to the SU controller.The IC 2312 of the subscriber line interface 2310 produces a digitalinformation signal TINF which is the input signal to the ADPCM encoder2321 if the user information signal is to be ADPCM encoded, such as forPOTS service. For data or other non-ADPCM encoded user information, theIC 2312 passes the data message TDMESS directly to the data interface2320. The call control module (CC), including the subscriber lineinterface 2310, derives call control information from the userinformation signal, and passes the call control information CCINF to theSU controller 2330. The ADPCM encoder 2321 also receives coding signaland bearer modification signals from the SU controller 2330 and convertsthe input digital information signal into the output message trafficsignal TVMESS in response to the coding and bearer modification signals.The SU controller 2330 also outputs the reverse control signal whichincludes the coding signal call control information, and bearer channelmodification signal, to the CDMA modem. The output message signal TVMESSis applied to the data interface 2320. The data interface 2320 sends theuser information to the CDMA modem 2340 as transmit message signalTMESS. The CDMA modem 2340 spreads the output message and reversecontrol channels TCTRL received from the SU controller 2330 and producesthe reverse link CDMA signal. The reverse link CDMA signal is providedto the RF transmit section 2301 and modulated by the RF modulator 2302to produce the output reverse link CDMA RF channel signal transmittedfrom antenna 2305.

XXVII. Call Connection and Establishment Procedure

The process of bearer channel establishment consists of two procedures:the call connection process for a call connection incoming from a remotecall processing unit such as an RDU (incoming call connection), and thecall connection process for a call outgoing from the SU (outgoing callconnection). Before any bearer channel can be established between an RCSand a SU, the SU must register its presence in the network with theremote call processor such as the RDU. When the off-hook signal isdetected by the SU, the SU not only begins to establish a bearerchannel, but also initiates the procedure for an RCS to obtain aterrestrial link between the RCS and the remote processor. Asincorporated herein by reference, the process of establishing the RCSand RDU connection is detailed in the DECT V5.1 standard.

For the incoming call connection procedure shown in FIG. 24, first 2401,the WAC 920 (shown in FIG. 9) receives, via one of the MUXs 905, 906 and907, an incoming call request from a remote call processing unit. Thisrequest identifies the target SU and that a call connection to the SU isdesired. The WAC periodically outputs the SBCH channel with pagingindicators for each SU and periodically outputs the FBCH traffic lightsfor each access channel. In response to the incoming call request, theWAC, at step 2420, first checks to see if the identified SU is alreadyactive with another call. If so, the WAC returns a busy signal for theSU to the remote processing unit through the MUX, otherwise the pagingindicator for the channel is set.

Next, at step 2402, the WAC checks the status of the RCS modems and, atstep 2421, determines whether there is an available modem for the call.If a modem is available, the traffic lights on the FBCH indicate thatone or more AXCH channels are available. If no channel is availableafter a certain period of time, then the WAC returns a busy signal forthe SU to the remote processing unit through the MUX. If an RCS modem isavailable and the SU is not active (in sleep mode), the WAC sets thepaging indicator for the identified SU on the SBCH to indicate anincoming call request. Meanwhile, the access channel modems continuouslysearch for the short access pilot signal (SAXPT) of the SU.

At step 2403, an SU in sleep mode periodically enters awake mode. Inawake mode, the SU modem synchronizes to the downlink pilot signal,waits for the SU modem AMF filters and phase locked loop to settle, andreads the paging indicator in the slot assigned to it on the SBCH todetermine if there is a call for the SU 2422. If no paging indicator isset, the SU halts the SU modem and returns to sleep mode. If a pagingindicator is set for an incoming call connection, the SU modem checksthe service type and traffic lights on FBCH for an available AXCH.

Next, at step 2404, the SU modem selects an available AXCH and starts afast transmit power ramp-up on the corresponding SAXPT. For a period theSU modem continues fast power ramp-up on SAXPT and the access modemscontinue to search for the SAXPT. At step 2405, the RCS modem acquiresthe SAXPT of the SU and begins to search for the SU LAXPT. When theSAXPT is acquired, the modem informs the WAC controller, and the WACcontroller sets the traffic lights corresponding to the modem to “red”to indicate the modem is now busy. The traffic lights are periodicallyoutput while continuing to attempt acquisition of the LAXPT.

The SU modem monitors, at step 2406, the FBCH AXCH traffic light. Whenthe AXCH traffic light is set to red, the SU assumes the RCS modem hasacquired the SAXPT and begins transmitting LAXPT. The SU modem continuesto ramp-up power of the LAXPT at a slower rate until sync-ind messagesare received on the corresponding CTCH. If the SU is mistaken becausethe traffic light was actually set in response to another SU acquiringthe AXCH, the SU modem times out because no sync-ind messages arereceived. The SU randomly waits a period of time, picks a new AXCHchannel, and steps 2404 and 2405 are repeated until the SU modemreceives sync-ind messages. Details of the power ramp-up method used inthe exemplary embodiment of this invention may be found in Section SUhereinafter entitled “Method Of Controlling Initial Power Ramp-Up InCDMA Systems By Using Short Codes.”

Next, at step 2407, the RCS modem acquires the LAXPT of the SU andbegins sending sync-ind messages on the corresponding CTCH. The modemwaits 10 msec for the pilot and AUX Vector correlator filters andphase-locked loop to settle, but continues to send sync-ind messages onthe CTCH. The modem then begins looking for a request message for accessto a bearer channel (MAC_ACC_REQ), from the SU modem.

The SU modem, at step 2408, receives the sync-ind message and freezesthe LAXPT transmit power level. The SU modem then begins sendingrepeated request messages for access to a bearer traffic channel(MAC_ACC_REQ) at fixed power levels, and listens for a requestconfirmation message (MAC_BEARER_CFM) from the RCS modem.

Next, at step 2409, the RCS modem receives a MAC_ACC_REQ message; themodem then starts measuring the AXCH power level, and starts the APCchannel. The RCS modem then sends the MAC_BEARER_CFM message to the SUand begins listening for the acknowledgment MAC_BEARER_CFM_ACK of theMAC_BEARER_CFM message. At step 2410, the SU modem receives theMAC_BEARER_CFM message and begins obeying the APC power controlmessages. The SU stops sending the MAC_ACC_REQ message and sends the RCSmodem the MAC_BEARER_CFM_ACK message. The SU begins sending the nulldata on the AXCH. The SU waits 10 msec for the uplink transmit powerlevel to settle.

The RCS modem, at step 2411, receives the MAC_BEARER_CFM_ACK message andstops sending the MAC_BEARER_CFM messages. APC power measurementscontinue.

Next, at step 2412, both the SU and the RCS modems have synchronized thesub-epochs, obey APC messages, measure receive power levels, and computeand send APC messages. The SU waits 10 msec for downlink power level tosettle.

Finally, at step 2413, the bearer channel is established and initializedbetween the SU and RCS modems. The WAC receives the bearer establishmentsignal from the RCS modem, re-allocates the AXCH channel and sets thecorresponding traffic light to green.

For the Outgoing Call Connection shown in FIG. 25, the SU is placed inactive mode by the off-hook signal at the user interface at step 2501.Next, at step 2502, the RCS indicates available AXCH channels by settingthe respective traffic lights. At step 2503, the SU synchronizes to thedownlink pilot, waits for the SU modem vector correlator filters andphase lock loop to settle, and the SU checks service type and trafficlights for an available AXCH. Steps 2504 through 2513 are identical tothe procedure steps 2404 through 2413 for the incoming call connectionprocedure of FIG. 24, and therefore are not explained in detail.

In the previous procedures for incoming call connection and outgoingcall connection, the power ramping-up process consists of the followingevents. The SU starts from very low transmit power and increases itspower level while transmitting the short code SAXPT; once the RCS modemdetects the short code it turns off the traffic light. Upon detectingthe changed traffic light, the SU continues ramping-up at a slower ratethis time sending the LAXPT. Once the RCS modem acquires the LAXPT andsends a message on CTCH to indicate this, the SU keeps its transmit (TX)power constant and sends the MAC-access-request message. This message isanswered with a MAC_BEARER_CFM message on the CTCH. Once the SU receivesthe MAC_BEARER_CFM message it switches to the traffic channel (TRCH)which is the dial tone for POTS.

When the SU captures a specific user channel AXCH, the RCS assigns acode seed for the SU through the CTCH. The code seed is used by thespreading code generator in the SU modem to produce the assigned codefor the reverse pilot of the SU, and the spreading codes for associatedchannels for traffic, call control, and signaling. The SU reverse pilotspreading code sequence is synchronized in phase to the RCS systemglobal pilot spreading code, and the traffic, call control and signalingspreading codes are synchronized in phase to the SU reverse pilotspreading code.

If the SU is successful in capturing a specific user channel, the RCSestablishes a terrestrial link with the remote processing unit tocorrespond to the specific user channel. For the DECT V5.1 standard,once the complete link from the RDU to the LE is established using theV5.1 ESTABLISHMENT message, a corresponding V5.1 ESTABLISHMENT ACKmessage is returned from the LE to the RDU, and the SU is sent a CONNECTmessage indicating that the transmission link is complete.

XXVIII. Support of Special Service Types

The system of the present invention includes a bearer channelmodification feature which allows the transmission rate of the userinformation to be switched from a lower rate to a higher rate. Thebearer channel modification (BCM) method is used to change a 32 kbsADPCM channel to a 64 kbs PCM channel to support high speed data and faxcommunications through the spread-spectrum communication system of thepresent invention. Although the details of this technique are describedin Section XXXXV hereinafter entitled “CDMA Communication System WhichSelectively Suppresses Data Transmissioning During Establishment Of ACommunication Channel”, the process is briefly described below:

First, a bearer channel on the RF interface is established between theRCS and SU, and a corresponding link exists between the RCS terrestrialinterface and the remote processing unit, such as an RDU. The digitaltransmission rate of the link between the RCS and remote processing unitnormally corresponds to a data encoded rate, which may be, for example,ADPCM at 32 kbs. The WAC controller of the RCS monitors the encodeddigital data information of the link received by the line interface ofthe MUX. If the WAC controller detects the presence of the 2100 Hz tonein the digital data, the WAC instructs the SU through the assignedlogical control channel and causes a second, 64 kbs duplex link to beestablished between the RCS modem and the SU. In addition, the WACcontroller instructs the remote processing unit to establish a second 64kbs duplex link between the remote processing unit and the RCS.Consequently, for a brief period, the remote processing unit and the SUexchange the same data over both the 32 kbs and the 64 kbs links throughthe RCS. Once the second link is established, the remote processing unitcauses the WAC controller to switch transmission only to the 64 kbslink, and the WAC controller instructs the RCS modem and the SU toterminate and tear down the 32 kbs link. Concurrently, the 32 kbsterrestrial link is also terminated and torn down.

Another embodiment of the BCM method incorporates a negotiation betweenthe external remote processing unit, such as the RDU, and the RCS toallow for redundant channels on the terrestrial interface, while onlyusing one bearer channel on the RF interface. The method described is asynchronous switchover from the 32 kbs link to the 64 kbs link over theair link which takes advantage of the fact that the spreading codesequence timing is synchronized between the RCS modem and SU. When theWAC controller detects the presence of the 2100 Hz tone in the digitaldata, the WAC controller instructs the remote processing unit toestablish a second 64 kbs duplex link between the remote processing unitand the RCS. The remote processing unit then sends 32 kbs encoded dataand 64 kbs data concurrently to the RCS. Once the remote processing unithas established the 64 kbs link, the RCS is informed and the 32 kbs linkis terminated and torn down. The RCS also informs the SU that the 32 kbslink is being torn down and to switch processing to receive unencoded 64kbs data on the channel. The SU and RCS exchange control messages overthe bearer control channel of the assigned channel group to identify anddetermine the particular subepoch of the bearer channel spreading codesequence within which the RCS will begin transmitting 64 kbit/sec datato the SU. Once the subepoch is identified, the switch occurssynchronously at the identified subepoch boundary. This synchronousswitchover method is more economical of bandwidth since the system doesnot need to maintain capacity for a 64 kbs link in order to support aswitchover.

In previously described embodiments of the BCM feature, the RCS willtear down the 32 kbs link first, but one skilled in the art would knowthat the RCS could tear down the 32 kbs link after the bearer channelhas switched to the 64 kbs link.

As another special service type, the system of the present inventionincludes a method for conserving capacity over the RF interface for ISDNtypes of traffic. This conservation occurs while a known idle bitpattern is transmitted in the ISDN D-channel when no data information isbeing transmitted. The CDMA system of the present invention includes amethod to prevent transmission of redundant information carried on theD-channel of ISDN networks for signals transmitted through a wirelesscommunication link. The advantage of such method is that it reduces theamount of information transmitted and consequently the transmit powerand channel capacity used by that information. The method is describedas it is used in the RCS. In the first step, the controller, such as theWAC of the RCS or the SU controller of the SU, monitors the outputD-channel from the subscriber line interface for a pre-determinedchannel idle pattern. A delay is included between the output of the lineinterface and the CDMA modem. Once the idle pattern is detected, thecontroller inhibits the transmission of the spread message channelthrough a message included in the control signal to the CDMA modem. Thecontroller continues to monitor the output D-channel of the lineinterface until the presence of data information is detected. When datainformation is detected, the spread message channel is activated.Because the message channel is synchronized to the associated pilotwhich is not inhibited, the corresponding CDMA modem of the other end ofthe communication link does not have to reacquire synchronization to themessage channel.

XXIX. Drop Out Recovery

The RCS and SU each monitor the CDMA bearer channel signal to evaluatethe quality of the CDMA bearer channel connection. Link quality isevaluated using the SPRT process employing adaptive quantile estimation.The SPRT process uses measurements of the received signal power and, ifthe SPRT process detects that the local spreading code generator haslost synchronization with the received signal spreading code or if itdetects the absence or low level of a received signal, the SPRT declaresLOL.

When the LOL condition is declared, the receiver modem of each RCS andSU begins a Z-search of the input signal with the local spreading codegenerator. Z-search is well known in the art of CDMA spreading codeacquisition and detection and is described in Digital Communications andSpread Spectrum Systems, by Robert E. Ziemer and Roger L. Peterson, atpages 492-94 which is incorporated herein by reference. The Z-searchalgorithm of the present invention tests groups of eight spreading codephases ahead and behind the last known phase in larger and largerspreading code phase increments.

During the LOL condition detected by the RCS, the RCS continues totransmit to the SU on the assigned channels, and continues to transmitpower control signals to the SU to maintain SU transmit power level. Themethod of transmitting power control signals is described below.Successful reacquisition desirably takes place within a specified periodof time. If reacquisition is successful, the call connection continues,otherwise the RCS tears down the call connection by deactivating anddeallocating the RCS modem assigned by the WAC, and transmits a calltermination signal to a remote call processor, such as the RDU, asdescribed previously.

When the LOL condition is detected by the SU, the SU stops transmissionto the RCS on the assigned channels which forces the RCS into a LOLcondition, and starts the reacquisition algorithm. If reacquisition issuccessful, the call connection continues, and if not successful, theRCS tears down the call connection by deactivating and deallocating theSU modem as described previously.

XXX. Power Control

The power control feature of the present invention is used to minimizethe amount of transmit power used by an RCS and the SUs of the system,and the power control subfeature that updates transmit power duringbearer channel connection is defined as APC. APC data is transferredfrom the RCS to an SU on the forward APC channel and from an SU to theRCS on the reverse APC channel. When there is no active data linkbetween the two, the maintenance power control (MPC) subfeature updatesthe SU transmit power.

Transmit power levels of forward and reverse assigned channels andreverse global channels are controlled by the APC algorithm to maintainsufficient signal power to interference noise power ratio (SIR) on thosechannels, and to stabilize and minimize system output power. The presentinvention uses a closed loop power control mechanism in which a receiverdecides that the transmitter should incrementally raise or lower itstransmit power. This decision is conveyed back to the respectivetransmitter via the power control signal on the APC channel. Thereceiver makes the decision to increase or decrease the transmitter'spower based on two error signals. One error signal is an indication ofthe difference between the measured and desired despread signal powers,and the other error signal is an indication of the average receivedtotal power.

As used in the described embodiment of the invention, the term near-endpower control is used to refer to adjusting the transmitter's outputpower in accordance with the APC signal received on the APC channel fromthe other end. This means the reverse power control for the SU andforward power control for the RCS; and the term far-end APC is used torefer to forward power control for the SU and reverse power control forthe RCS (adjusting the opposite end's transmit power).

In order to conserve power, the SU modem terminates a transmission andpowers-down while waiting for a call, defined as the sleep phase. Sleepphase is terminated by an awaken signal from the SU controller. The SUmodem acquisition circuit automatically enters the reacquisition phaseand begins the process of acquiring the downlink pilot, as describedpreviously.

XXXI. Closed Loop Power Control Algorithms

The near-end power control consists of two steps: first, the initialtransmit power is set; and second, the transmit power is continuallyadjusted according to information received from the far-end using APC.

For the SU, initial transmit power is set to a minimum value and thenramped up, for example, at a rate of 1 dB/ms until either a ramp-uptimer expires (not shown) or the RCS changes the corresponding trafficlight value on the FBCH to “red” indicating that the RCS has locked tothe SU's short pilot SAXPT. Expiration of the timer causes the SAXPTtransmission to be shut down, unless the traffic light value is set tored first, in which case the SU continues to ramp-up transmit power butat a much lower rate than before the “red” signal was detected.

For the RCS, initial transmit power is set at a fixed value,corresponding to the minimum value necessary for reliable operation asdetermined experimentally for the service type and the current number ofsystem users. Global channels, such as global pilot or, FBCH, are alwaystransmitted at the fixed initial power, whereas traffic channels areswitched to APC.

The APC bits are transmitted as one bit up or down signals on the APCchannel. In the described embodiment, the 64 kbs APC data stream is notencoded or interleaved. Far-end power control consists of the near-endtransmitting power control information for the far-end to use inadjusting its transmit power. The APC algorithm causes the RCS or the SUto transmit +1 if the following inequality holds, otherwise −1.α₁ e ₁−α₂ e ₂>0  Equation (45)Here, the error signal e₁ is calculated as:e ₁ =P _(d)−(1+SNR_(REQ))P _(N)  Equation (46)where P_(d) is the despread signal plus noise power, P_(N) is thedespread noise power, and SNR_(REQ) is the desired despread signal tonoise ratio for the particular service type; and:e ₂ =P _(r) −P _(o)  Equation (47)where P_(r) is a measure of the received power and P_(o) is the AGCcircuit set point. The weights α₁ and α₂ in Equation (45) are chosen foreach service type and APC update rate.

XXXII. Maintenance Power Control

During the sleep phase of the SU, the interference noise power of theCDMA RF channel may change. The present invention includes a maintenancepower control feature (MPC) which periodically adjusts the SU's initialtransmit power with respect to the interference noise power of the CDMAchannel. The MPC is the process whereby the transmit power level of anSU is maintained within close proximity of the minimum level for the RCSto detect the SU's signal. The MPC process compensates for low frequencychanges in the required SU transmit power.

The maintenance control feature uses two global channels: one is calledthe status channel (STCH) on reverse link, and the other is called thecheck-up channel (CUCH) on forward link. The signals transmitted onthese channels carry no data and they are generated the same way theshort codes used in initial power ramp-up are generated. The STCH andCUCH codes are generated from a “reserved” branch of the global codegenerator.

The MPC process is as follows. At random intervals, the SU sends asymbol length spreading code periodically for 3 ms on the status channel(STCH). If the RCS detects the sequence, it replies by sending a symbollength code sequence within the next 3 ms on the check-up channel(CUCH). When the SU detects the response from the RCS, it reduces itstransmit power by a particular step size. If the SU does not see anyresponse from the RCS within that 3 ms period, it increases its transmitpower by the step size. Using this method, the RCS response istransmitted at a power level that is enough to maintain a 0.99 detectionprobability at all SU's.

The rate of change of traffic load and the number of active users isrelated to the total interference noise power of the CDMA channel. Theupdate rate and step size of the maintenance power update signal for thepresent invention is determined by using queuing theory methods wellknown in the art of communication theory, such as outlined in“Fundamentals of Digital Switching” (Plenum-New York) edited by McDonaldand incorporated herein by reference. By modeling the call originationprocess as an exponential random variable with mean 6.0 mins, numericalcomputation shows the maintenance power level of a SU should be updatedonce every 10 seconds or less to be able to follow the changes ininterference level using 0.5 dB step size. Modeling the call originationprocess as a Poisson random variable with exponential interarrivaltimes, arrival rate of 2×10⁻⁴ per second per user, service rate of 1/360per second, and the total subscriber population is 600 in the RCSservice area also yields by numerical computation that an update rate ofonce every 10 seconds is sufficient when 0.5 dB step size is used.

Maintenance power adjustment is performed periodically by the SU whichchanges from sleep phase to awake phase and performs the MPC process.Consequently, the process for the MPC feature is shown in FIG. 26 and isas follows: First, at step 2601, signals are exchanged between the SUand the RCS maintaining a transmit power level that is close to therequired level for detection: the SU periodically sends a symbol lengthspreading code in the STCH and the RCS periodically sends a symbollength spreading code in the CUCH as response.

Next, at step 2602, if the SU receives a response within 3 ms after theSTCH message it sent, it decreases its transmit power by a particularstep size at step 2603; but if the SU does not receive a response within3 ms after the STCH message, it increases its transmit power by the samestep size at step 2604.

The SU waits, at step 2605, for a period of time before sending anotherSTCH message, this time period is determined by a random process whichaverages 10 seconds. Thus, the transmit power of the STCH messages fromthe SU is adjusted based on the RCS response periodically, and thetransmit power of the CUCH messages from the RCS is fixed (step 2606).

XXXIII. Mapping of Power Control Signal to Logical Channels for APC

Power control signals are mapped to specified logical channels forcontrolling transmit power levels of forward and reverse assignedchannels. Reverse global channels are also controlled by the APCalgorithm to maintain sufficient signal power to interference noisepower ratio (SIR) on those reverse channels, and to stabilize andminimize system output power. The present invention uses a closed looppower control method in which a receiver periodically decides toincrementally raise or lower the output power of the transmitter at theother end. The method also conveys that decision back to the respectivetransmitter.

TABLE 13 APC Signal Channel Assignments Link Channels andCall/Connection Power Control Method Signals Status Initial ValueContinuous Reverse link being established as determined by APC bits inAXCH power ramping forward APC AXPT channel Reverse link in-progresslevel established APC bits in APC, OW, during call set-up forward APCTRCH, channel pilot signal Forward link in-progress fixed value APC bitsin APC, OW, reverse APC TRCH channel

Forward and reverse links are independently controlled. For acall/connection in process, forward link (TRCHs, APC, and OW) power iscontrolled by the APC bits transmitted on the reverse APC channel.During the call/connection establishment process, reverse link (AXCH)power is also controlled by the APC bits transmitted on the forward APCchannel. Table 13 summarizes the specific power control methods for thecontrolled channels.

The required SIRs of the assigned channels TRCH, APC and OW and reverseassigned pilot signal for any particular SU are fixed in proportion toeach other and these channels are subject to nearly identical fading,therefore, they are power controlled together.

XXXIV. AFPC

The AFPC process attempts to maintain the minimum required SIR on theforward channels during a call/connection. The AFPC recursive process,shown in FIG. 27, consists of the steps of having an SU form the twoerror signals e₁ and e₂ in step 2701 where:e ₁ =P _(d)−(1+SNR_(REQ))P _(N)  Equation (48)e ₂ =P _(r) −P _(o)  Equation (49)and P_(d) is the despread signal plus noise power, P_(N) is the despreadnoise power, SNR_(REQ) is the required signal to noise ratio for theservice type, P_(r) is a measure of the total received power, and P_(o)is the AGC set point. Next, the SU modem forms the combined error signalα₁e₁+α₂e₂ in step 2702. Here, the weights α₁ and α₂ are chosen for eachservice type and APC update rate. In step 2703, the SU hard limits thecombined error signal and forms a single APC bit. The SU transmits theAPC bit to the RCS in step 2704 and RCS modem receives the bit in step2705. The RCS increases or decreases its transmit power to the SU instep 2706 and the algorithm repeats starting from step 2701.

XXXV. ARPC

The ARPC process maintains the minimum desired SIR on the reversechannels to minimize the total system reverse output power, during bothcall/connection establishment and while the call/connection is inprogress. The recursive ARPC process, shown in FIG. 28, begins at step2801 where the RCS modem forms the two error signals e₁ and e₂ in step2801 where:e ₁ =P _(d)−(1+SNR_(REQ))P _(N)  Equation (50)e ₂ =P _(rt) −P _(o)  Equation (51)and P_(d) is the despread signal plus noise power, P_(N) is the despreadnoise power, SNR_(REQ) is the desired signal to noise ratio for theservice type, P_(rt) is a measure of the average total power received bythe RCS, and P_(o) is the AGC set point. The RCS modem forms thecombined error signal α₁e₁+α₂e₂ in step 2802 and hard limits this errorsignal to determine a single APC bit in step 2803. The RCS transmits theAPC bit to the SU in step 2804, and the bit is received by the SU instep 2805. Finally, the SU adjusts its transmit power according to thereceived APC bit in step 2806, and the algorithm repeats starting fromstep 2801.

TABLE 14 Symbols/Thresholds Used for APC Computation Call/ConnectionSymbol (and Threshold) Used Service or Call Type Status for APC DecisionDon't care being AXCH established ISDN D SU in-progress one 1/64-kbssymbol from TRCH (ISDN-D) ISDN 1B + D SU in-progress TRCH (ISDN-B) ISDN2B + D SU in-progress TRCH (one ISDN-B) POTS SU (64 KBPS in-progress one1/64-KBPS symbol from PCM) TRCH, use 64 KBPS PCM threshold POTS SU (32KBPS in-progress one 1/64-KBPS symbol from ADPCM) TRCH, use 32 KBPSADPCM threshold Silent Maintenance in-progress OW (continuous during aCall (any SU) maintenance call)

XXXVI. SIR and Multiple Channel Types

The required SIR for channels on a link is a function of channel format(e.g. TRCH, OW), service type (e.g. ISDN B, 32 KBPS ADPCM POTS), and thenumber of symbols over which data bits are distributed (e.g. two 64 kbssymbols are integrated to form a single 32 kbs ADPCM POTS symbol).Despreader output power corresponding to the required SIR for eachchannel and service type is predetermined. While a call/connection is inprogress, several user CDMA logical channels are concurrently active;each of these channels transfers a symbol every symbol period. The SIRof the symbol from the nominally highest SIR channel is measured,compared to a threshold and used to determine the APC step up/downdecision each symbol period. Table 14 indicates the symbol (andthreshold) used for the APC computation by service and call type.

XXXVII. APC Parameters

APC information is always conveyed as a single bit of information, andthe APC data rate is equivalent to the APC update rate. The APC updaterate is 64 kbs. This rate is high enough to accommodate expectedRayleigh and Doppler fades and allow for a relatively high (˜0.2) biterror rate (BER) in the uplink and downlink APC channels, whichminimizes capacity devoted to the APC.

The power step up/down indicated by an APC bit is nominally between 0.1and 0.01 dB. The dynamic range for power control is 70 dB on the reverselink and 12 dB on the forward link for the exemplary embodiment of thepresent system.

XXXVIII. An Alternative Embodiment of Multiplexing of APC Information

The dedicated APC and OW logical channels described previously can alsobe multiplexed together in one logical channel. The APC information istransmitted at 64 kbs continuously whereas the OW information occurs indata bursts. The alternative multiplexed logical channel includes theunencoded, non-interleaved 64 kbs. APC information on, for example, thein-phase channel and the OW information on the quadrature channel of theQPSK signal.

XXXIX. Closed Loop Power Control Implementation

The closed loop power control during a call connection responds to twodifferent variations in overall system power. First, the system respondsto local behavior such as changes in power level of an SU, and second,the system responds to changes in the power level of the entire group ofactive users in the system.

The APC system of the exemplary embodiment of the present invention isshown in FIG. 29. As shown, the circuitry used to adjust the transmittedpower is similar for the RCS (shown as the RCS power control module2901) and SU (shown as the SU power control module 2902). Beginning withthe RCS power control module 2901, the reverse link RF channel signal isreceived at the RF antenna and demodulated to produce the reverse CDMAsignal RMCH. The signal RMCH is applied to the variable gain amplifier(VGA1) 2910 which produces an input signal to the AGC circuit 2911. TheAGC 2911 produces a variable gain amplifier control signal into the VGA12910. This signal maintains the level of the output signal of VGA1 2910at a near constant value. The output signal of VGA1 is despread by thedespread-demultiplexer (demux) 2912, which produces a despread usermessage signal MS and a forward APC bit. The forward APC bit is appliedto the integrator 2913 to produce the forward APC control signal. Theforward APC control signal controls the forward link VGA2 2914 andmaintains the forward link RF channel signal at a minimum desired levelfor communication.

The signal power of the despread user message signal MS of the RCS powermodule 2901 is measured by the power measurement circuit 2915 to producea signal power indication. The output of the VGA1 is also despread bythe AUX despreader 2981 which despreads the signal by using anuncorrelated spreading code, and hence obtains a despread noise signal.The power measurement by measure power 2982 of this signal is multipliedat multiplier 2983 by 1 plus the desired signal to noise ratio (SNR_(R))to form the threshold signal S1. The difference between the despreadsignal power and the threshold value S1 is produced by the subtracter2916. This difference is the error signal ES1, which is an error signalrelating to the particular SU transmit power level. Similarly, thecontrol signal for the VGA1 2910 is applied to the rate scaling circuit2917 to reduce the rate of the control signal for VGA1 2910. The outputsignal of scaling circuit 2917 is a scaled system power level signalSP1. The threshold compute logic 2918 calculates the system signalthreshold value SST from the RCS user channel power data signal RCSUSR.The complement of the scaled system power level signal, SP1, and thesystem signal power threshold value SST are applied to the adder 2919which produces the second error signal ES2. This error signal is relatedto the system transmit power level of all active SUs. The input errorsignals ES1 and ES2 are combined in the combiner 2920 to produce acombined error signal input to the delta modulator (DM1) 2921, and theoutput signal of the DM1 is the reverse APC bit stream signal, havingbits of value +1 or −1, which for the present invention is transmittedas a 64 kbs signal.

The reverse APC bit is applied to the spreading circuit 2922, and theoutput signal of the spreading circuit 2922 is the spread-spectrumforward APC message signal. Forward OW and traffic signals are alsoprovided to spreading circuits 2923, 2924, producing forward trafficmessage signals 1, 2, . . . N. The power level of the forward APCsignal, the forward OW, and traffic message signals are adjusted by therespective amplifiers 2925, 2926 and 2927 to produce the power leveladjusted forward APC, OW and TRCH channels signals. These signals arecombined by the adder 2928 and applied to the VGA2 2914, which producesforward link RF channel signal.

The forward link RF channel signal including the spread forward APCsignal is received by the RF antenna of the SU, and demodulated toproduce the forward CDMA signal FMCH. This signal is provided to thevariable gain amplifier (VGA3) 2940. The output signal of VGA3 isapplied to the AGC 2941 which produces a variable gain amplifier controlsignal to VGA3 2940. This signal maintains the level of the outputsignal of VGA3 at a near constant level. The output signal of VGA3 2940is despread by the despread demux 2942, which produces a despread usermessage signal SUMS and a reverse APC bit. The reverse APC bit isapplied to the integrator 2943 which produces the reverse APC controlsignal. This reverse APC control signal is provided to the reverse APCVGA4 2944 to maintain the reverse link RF channel signal at a minimumpower level.

The despread user message signal SUMS is also applied to the powermeasurement circuit 2945 producing a power measurement signal, which isadded to the complement of threshold value S2 in the adder 2946 toproduce error signal ES3. The signal ES3 is an error signal relating tothe RCS transmit power level for the particular SU. To obtain thresholdS2, the noise power of a despread signal produced by the AUX despreader2985, as measured by the power measurement circuit 2986, is multipliedusing multiplier 2987 by 1 plus the desired signal to noise ratioSNR_(R). The AUX despreader 2985 despreads the output signal of VGA3using an uncorrelated spreading code, hence its output is an indicationof the despread noise power. Similarly, the control signal for the VGA3is applied to the rate scaling circuit 2970 to reduce the rate of thecontrol signal for VGA3 in order to produce a scaled received powerlevel RP1. The threshold compute circuit 2998 computes the receivedsignal threshold RST from the SU measured power signal SUUSR. Thecomplement of the scaled received power level RP1 and the receivedsignal threshold RST are applied to the adder 2994 which produces errorsignal ES4. This error is related to the RCS transmit power to all otherSUs. The input error signals ES3 and ES4 are combined in the combiner2999 and input to the delta modulator DM2 2947. The output signal of DM22947 is the forward APC bit stream signal, with bits having value ofvalue +1 or −1. In the exemplary embodiment of the present invention,this signal is transmitted as a 64 kbs signal.

The forward APC bit stream signal is applied to the spreading circuit2948, to produce the output reverse spread-spectrum APC signal. ReverseOW and traffic signals are also input to spreading circuits 2949, 2950,producing reverse OW and traffic message signals 1, 2, . . . N, and thereverse pilot is generated by the reverse pilot generator 2951. Thepower level of the reverse APC message signal, reverse OW messagesignal, reverse pilot, and the reverse traffic message signals areadjusted by amplifiers 2952, 2953, 2954, 2955 to produce the signalswhich are combined by the adder 2956 and input to the reverse APC VGA42944. It is this VGA4 2944 which produces the reverse link RF channelsignal.

During the call connection and bearer channel establishment process, theclosed loop power control of the present invention is modified, and isshown in FIG. 30. As shown, the circuits used to adjust the transmittedpower are different for the RCS, shown as the initial RCS power controlmodule 3001; and for the SU, shown as the initial SU power controlmodule 3002. Beginning with the initial RCS power control module 3001,the reverse link RF channel signal is received at the RF antenna anddemodulated producing the reverse CDMA signal IRMCH which is received bythe first variable gain amplifier (VGA1) 3003. The output signal of VGA1is detected by the AGC circuit (AGC1) 3004 which provides a variablegain amplifier control signal to VGA1 3003 to maintain the level of theoutput signal of VGA1 at a near constant value. The output signal ofVGA1 is despread by the despread demultiplexer 3005, which produces adespread user message signal IMS. The forward APC control signal, ISET,is set to a fixed value, and is applied to the forward link variablegain amplifier (VGA2) 3006 to set the forward link RF channel signal ata predetermined level.

The signal power of the despread user message signal IMS of the InitialRCS power module 3001 is measured by the power measure circuit 3007, andthe output power measurement is subtracted from a threshold value S3 inthe subtracter 3008 to produce error signal ES5, which is an errorsignal relating to the transmit power level of a particular SU. Thethreshold S3 is calculated by multiplying using a multiplier 3083 thedespread power measurement by measure power 3082 obtained from the AUXdespreader 3081 by 1 plus the desired signal to noise ratio SNR_(R). TheAUX despreader 3081 despreads the signal using an uncorrelated spreadingcode, hence its output signal is an indication of despread noise power.Similarly, the VGA1 control signal is applied to the rate scalingcircuit 3009 to reduce the rate of the VGA1 control signal in order toproduce a scaled system power level signal SP2. The thresholdcomputation logic 3010 determines an initial system signal thresholdvalue (ISST) computed from the user channel power data signal (IRCSUSR).The complement of the scaled system power level signal SP2 and the ISSTare provided to the adder 3011 which produces a second error signal ES6,which is an error signal relating to the system transmit power level ofall active SUs. The value of ISST is the desired transmit power for asystem having the particular configuration. The input error signals ES5and ES6 are combined in the combiner 3012 to produce a combined errorsignal input to the delta modulator (DM3) 3013. DM3 produces the initialreverse APC bit stream signal, having bits of value +1 or −1, which inthe exemplary embodiment is transmitted as a 64 kbs signal.

The reverse APC bit stream signal is applied to the spreading circuit3014, to produce the initial spread-spectrum forward APC signal. TheCTCH information is spread by the spreader 3016 to form the spread CTCHmessage signal. The spread APC and CTCH signals are scaled by theamplifiers 3015 and 3017, and combined by the combiner 3018. Thecombined signal is applied to VGA2 3006, which produces the forward linkRF channel signal.

The forward link RF channel signal including the spread forward APCsignal is received by the RF antenna of the SU and demodulated toproduce the initial forward CDMA signal (IFMCH) which is applied to thevariable gain amplifier (VGA3) 3020. The output signal of VGA3 isdetected by the AGC circuit (AGC2) 3021 which produces a variable gainamplifier control signal for the VGA3 3020. This signal maintains theoutput power level of the VGA3 3020 at a near constant value. The outputsignal of VGA3 is despread by the despread demultiplexer 3022, whichproduces an initial reverse APC bit that is dependent on the outputlevel of VGA3. The reverse APC bit is processed by the integrator 3023to produce the reverse APC control signal. The reverse APC controlsignal is provided to the reverse APC VGA4 3024 to maintain the reverselink RF channel signal at a defined power level.

The global channel AXCH signal is spread by the spreading circuits 3025to provide the spread AXCH channel signal. The reverse pilot generator3026 provides a reverse pilot signal, and the signal power of AXCH andthe reverse pilot signal are adjusted by the respective amplifiers 3027and 3028. The spread AXCH channel signal and the reverse pilot signalare summed by the adder 3029 to produce reverse link CDMA signal. Thereverse link CDMA signal is received by the reverse APC VGA4 3024, whichproduces the reverse link RF channel signal output to the RFtransmitter.

XXXX. System Capacity Management

The system capacity management algorithm of the present inventionoptimizes the maximum user capacity for an RCS area, called a cell. Whenthe SU comes within a certain value of maximum transmit power, the SUsends an alarm message to the RCS. The RCS sets the traffic lights whichcontrol access to the system, to “red” which, as previously described,is a flag that inhibits access by the SU's. This condition remains ineffect until the call to the alarming SU terminates, or until thetransmit power of the alarming SU, measured at the SU, is a value lessthan the maximum transmit power. When multiple SUs send alarm messages,the condition remains in effect until either all calls from alarming SUsterminate or until the transmit power of the alarming SU, measured atthe SU, is less than the maximum transmit power. An alternativeembodiment monitors the bit error rate measurements from the FECdecoder, and holds the RCS traffic lights at “red” until the bit errorrate is less than a predetermined value.

The blocking strategy of the present invention includes a method whichuses the power control information transmitted from the RCS to an SU,and the received power measurements at the RCS. The RCS measures itstransmit power level, detects that a maximum value is reached anddetermines when to block new users. An SU preparing to enter the systemblocks itself if the SU reaches the maximum transmit power beforesuccessful completion of a bearer channel assignment.

Each additional user in the system has the effect of increasing thenoise level for all other users, which decreases the signal to noiseratio (SNR) that each user experiences. The power control algorithmmaintains a desired SNR for each user. Therefore, in the absence of anyother limitations, addition of a new user into the system has only atransient effect and the desired SNR is regained.

The transmit power measurement at the RCS is done by measuring eitherthe root mean square (rms) value of the baseband combined signal or bymeasuring the transmit power of the RF signal and feeding it back todigital control circuits. The transmit power measurement may also bemade by the SUs to determine if the unit has reached its maximumtransmit power. The SU transmit power level is determined by measuringthe control signal of the RF amplifier, and scaling the value based onthe service type, such as POTS, FAX, or ISDN.

The information that an SU has reached the maximum power is transmittedto the RCS by the SU in a message on the assigned channels. The RCS alsodetermines the condition by measuring reverse APC changes because, ifthe RCS sends APC messages to the SU to increase SU transmit power, andthe SU transmit power measured at the RCS is not increased, the SU hasreached the maximum transmit power.

The RCS does not use traffic lights to block new users who have finishedramping-up using the short codes. These users are blocked by denyingthem the dial tone and letting them time out. The RCS sends all 1's (godown commands) on the APC channel to make the SU lower its transmitpower. The RCS also sends either no CTCH message or a message with aninvalid address which would force the FSU to abandon the accessprocedure and start over. The SU, however, does not start theacquisition process immediately because the traffic lights are red.

When the RCS reaches its transmit power limit, it enforces blocking inthe same manner as when an SU reaches its transmit power limit. The RCSturns off all the traffic lights on the FBCH, starts sending all 1 APCbits (go down commands) to those users who have completed their shortcode ramp-up but have not yet been given a dial tone, and either sendsno CTCH message to these users or sends messages with invalid addressesto force them to abandon the access process.

The self blocking process of the SU is as follows. When the SU startstransmitting the AXCH, the APC starts its power control operation usingthe AXCH and the SU transmit power increases. While the transmit poweris increasing under the control of the APC it is monitored by the SUcontroller. If the transmit power limit is reached, the SU abandons theaccess procedure and starts over.

XXXXI. System Synchronization

The RCS is synchronized either to the PSTN network clock signal throughone of the line interfaces, as shown in FIG. 10 or to the RCS systemclock oscillator, which free-runs to provide a master timing signal forthe system. The global pilot channel, and therefore all logical channelswithin the CDMA channel, are synchronized to the system clock signal ofthe RCS. The global pilot (GLPT) is transmitted by the RCS and definesthe timing at the RCS transmitter.

The SU receiver is synchronized to the GLPT, and so behaves as a slaveto the network clock oscillator. However, the SU timing is retarded bythe propagation delay. In the present embodiment of the invention, theSU modem extracts a 64 KHz and 8 KHz clock signal from the CDMA RFreceive channel, and a PLL oscillator circuit creates 2 MHz and 4 MHzclock signals

The SU transmitter and hence the LAXPT or ASPT are slaved to the timingof the SU receiver. The RCS receiver is synchronized to the LAXPT or theASPT transmitted by the SU, however, its timing may be retarded by thepropagation delay. Hence, the timing of the RCS receiver is that of theRCS transmitter retarded by twice the propagation delay.

Furthermore, the system can be synchronized via a reference receivedfrom a GPS receiver. In a system of this type, a GPS receiver in eachRCS provides a reference clock signal to all submodules of the RCS.Because each RCS receives the same time reference from the GPS, all ofthe system clock signals in all of the RCSs are synchronized.

The present invention also performs multichannel filtering. Details ofthis technique can be found in Section XXXXVI hereinafter entitled“Efficient Multichannel Filtering For CDMC Modems”.

XXXXII. A Method of Controlling Initial Power Ramp-Up in CDMA Systems byUsing Short Codes

The use of the same frequency spectrum by a plurality of SUs increasesthe efficiency of a CDMA communication system. However, it also causes agradual degradation of the performance of the system as the number ofSUs increase. Each SU detects communication signals with its uniquespreading code as valid signals and all other signals are viewed asnoise. The stronger the signal from an SU arrives at the BS, the moreinterference the BS experiences when receiving and demodulating signalsfrom other SUs. Ultimately, the power from one SU may be great enough toterminate communications of other SUs. Accordingly, it is extremelyimportant in wireless CDMA communication systems to control thetransmission power of all SUs. The control of transmission power isparticularly critical when an SU is attempting to initiatecommunications with a BS and a power control loop has not yet beenestablished. Typically, the transmission power required from an SUchanges continuously as a function of the propagation loss, interferencefrom other SUs, channel noise, fading and other channel characteristics.Therefore, an SU does not know the power level at which it should starttransmitting. If the SU begins transmitting at a power level that is toohigh, it may interfere with the communications of other SUs and may eventerminate the communications of other SUs. If the initial transmissionpower level is too low, the SU will not be detected by the BS and acommunication link will not be established.

The present invention comprises a novel method of controllingtransmission power during the establishment of a channel in a CDMAcommunication system by utilizing the transmission of a short code froman SU to a BS during initial power ramp-up. The short code is a sequencefor detection by the BS which has a much shorter period than aconventional spreading code. The ramp-up starts from a power level thatis guaranteed to be lower than the required power level for detection bythe BS. The SU quickly increases transmission power while repeatedlytransmitting the short code until the signal is detected by the BS. Oncethe BS detects the short code, it sends an indication to the SU to ceaseincreasing transmission power. The use of short codes limits powerovershoot and interference to other SUs and permits the BS to quicklysynchronize to the spreading code used by the SU.

A communication network 3110 in one embodiment of the present inventionis shown in FIG. 31. The communication network 3110 generally comprisesone or more BSs 3114, each of which is in wireless communication with aplurality of SUs 3116, which may be fixed or mobile. Each SU 3116communicates with either the closest BS 3114 or the BS 3114 whichprovides the strongest communication signal. The BSs 3114 alsocommunicate with a base station controller 3120, which coordinatescommunications among BSs 3114. The communication network 3110 may alsobe connected to a local exchange (LE) 3122, wherein the base stationcontroller 3120 also coordinates communications between the BSs 3114 andthe LE 3122. Preferably, each BS 3114 communicates with the base stationcontroller 3120 over a wireless link, although a land line may also beprovided. A land line is particularly applicable when a BS 3114 is inclose proximity to the base station controller 3120.

The base station controller 3120 performs several functions. Primarily,the base station controller 3120 provides all of the operations,administrative and maintenance (OA&M) signaling associated withestablishing and maintaining all of the wireless communications betweenthe SUs 3116, the BSs 3114 and the base station controller 3120. Thebase station controller 3120 also provides an interface between thewireless communication system 3110 and the LE 3122. This interfaceincludes multiplexing and demultiplexing of the communication signalsthat enter and leave the system 3110 via the base station controller3120. Although the wireless communication system 3110 is shown employingantennas to transmit RF signals, one skilled in the art should recognizethat communications may be accomplished via microwave or satelliteuplinks. Additionally, the functions of the base station controller 3120may be combined with a BS 3114 to form a “master base station”.

Referring to FIG. 32, the propagation of signals between a BS 3114 and aplurality of SUs 3116 is shown. A two-way communication channel (link)3118 comprises a signal transmitted (Tx) 3126 from the BS 3114 to the SU3116 and a signal received (Rx) 3128 by the BS 3114 from the SU 3116.The Tx signal 3126 is transmitted from the BS 3114 and is received bythe SU 3116 after a propagation delay Δt. Similarly, the Rx signal 3128originates at the SU 3116 and terminates at the BS 3114 after a furtherpropagation delay Δt. Accordingly, the round trip propagation delay is2Δt. In the preferred embodiment, the BS 3114 has an operating range ofapproximately 30 kilometers. The round trip propagation delay 3124associated with an SU 3116 at the maximum operating range is 200microseconds.

It should be apparent to those of skill in the art that theestablishment of a communication channel between a BS and an SU is acomplex procedure as hereinbefore described involving many tasksperformed by the BS and the SU which are outside the scope of thepresent invention. This aspect of present invention is directed toinitial power ramp-up and synchronization during the establishment of acommunication channel.

Referring to FIG. 33, the signaling between a BS 3114 and an SU 3116 isshown. In accordance with the present invention, the BS 3114continuously transmits a pilot code 3140 to all of the SUs 3116 locatedwithin the transmitting range of the BS 3114. The SU 3116 must acquirethe pilot code 3140 transmitted by the BS 3114 before it can receive ortransmit any data. Acquisition is the process whereby the SU 3116 alignsits locally generated spreading code with the received pilot code 3140.The SU 3116 searches through all of the possible phases of the receivedpilot code 3140 until it detects the correct phase, (the beginning ofthe pilot code 3140).

The SU 3116 then synchronizes its transmit spreading code to thereceived pilot code 3140 by aligning the beginning of its transmitspreading code to the beginning of the pilot code 3140. One implicationof this receive and transmit synchronization is that the SU 3116introduces no additional delay as far as the phase of the spreadingcodes are concerned. Accordingly, as shown in FIG. 33, the relativedelay between the pilot code 3140 transmitted from the BS 3114 and theSU's transmit spreading code 3142 received at the BS 3114 is 2Δt, whichis solely due to the round trip propagation delay.

In the preferred embodiment, the pilot code is 29,877,120 chips inlength and takes approximately 2 to 5 seconds to transmit, depending onthe spreading factor. The length of the pilot code 3140 was chosen to bea multiple of the data symbol no matter what kind of data rate orbandwidth is used. As is well known by those of skill in the art, alonger pilot code 3140 has better randomness properties and thefrequency response of the pilot code 3140 is more uniform. Additionally,a longer pilot code 3140 provides low channel cross correlation, thusincreasing the capacity of the system 3110 to support more SUs 3116 withless interference. The use of a long pilot code 3140 also supports agreater number of random short codes. For synchronization purposes, thepilot code 3140 is chosen to have the same period as all of the otherspreading codes used by the system 3110. Thus, once a SU 3116 acquiresthe pilot code 3140, it is synchronized to all other signals transmittedfrom the BS 3114.

During idle periods, when a call is not in progress or pending, the SU3116 remains synchronized to the BS 3114 by periodically reacquiring thepilot code 3140. This is necessary for the SU 3116 to receive anddemodulate any downlink transmissions, in particular paging messageswhich indicate incoming calls.

When a communication link is desired, the BS 3114 must acquire thesignal transmitted from the SU 3116 before it can demodulate the data.The SU 3116 must transmit an uplink signal for acquisition by the BS3114 to begin establishing the two-way communication link. A criticalparameter in this procedure is the transmission power level of the SU3116. A transmission power level that is too high can impaircommunications in the whole service area, whereas a transmission powerlevel that is too low can prevent the BS 3114 from detecting the uplinksignal.

The SU 3116 starts transmitting at a power level guaranteed to be lowerthan what is required and increases transmission power output until thecorrect power level is achieved. This avoids sudden introduction of astrong interference, hence improving system 3110 capacity.

The establishment of a communication channel in accordance with thisembodiment of the present invention and the tasks performed by the BS3114 and an SU 3116 are shown in FIG. 34. Although many SUs 3116 may belocated within the operating range of the BS 3114, reference will bemade hereinafter to a single SU 3116 for simplicity in explaining theoperation of the present invention. Additionally, although theterminology “access code” is used herein as referring to the spreadingcode used with the “access signal”, access code and access signal may beused interchangeably. Finally, the terminology “confirmation signal” and“acknowledgement signal” may also be used interchangeably.

The BS 3114 begins by continuously transmitting a periodic pilot code3140 to all SUs 3116 located within the operating range of the BS 3114(step 3400). As the BS 3114 transmits the pilot code 3140, the BS 3114searches for an “access code” 3142 transmitted by an SU 3116 (step3402). The access code 3142 is a known spreading code transmitted froman SU 3116 to the BS 3114 during initiation of communications and powerramp-up. The BS 3114 must search through all possible phases (timeshifts) of the access code 3142 transmitted from the SU 3116 in order tofind the correct phase. This is called the “acquisition” or the“detection” process. The longer the access code 3142, the longer ittakes for the BS 3114 to search through the phases and acquire thecorrect phase.

As previously explained, the relative delay between signals transmittedfrom the BS 3114 and return signals received at the BS 3114 correspondsto the round trip propagation delay 2Δt. The maximum delay occurs at themaximum operating range of the BS 3114, known as the cell boundary.Accordingly, the BS 3114 must search up to as many code phases as thereare in the maximum round trip propagation delay, which is typically lesscode phases than there are in a code period.

For a data rate Rb and spreading code rate Rc, the ratio L=Rc/Rb iscalled the spreading factor or the processing gain. In the preferredembodiment of the present invention, the cell boundary radius is 30 km,which corresponds to approximately between 1000 and 2500 code phases inthe maximum round trip delay, depending on the processing gain.

If the BS 3114 has not detected the access code after searching throughthe code phases corresponding to the maximum round trip delay, thesearch is repeated starting from the phase of the pilot code 3140 whichcorresponds to zero delay (step 3404).

During idle periods, the pilot code 3140 from the BS 3114 is received atthe subscriber unit 3116 which periodically synchronizes its transmitspreading code generator thereto (step 3406). If synchronization withthe pilot code 3140 is lost, the SU 3116 reacquires the pilot code 3140and resynchronizes (step 3408).

When it is desired to initiate a communication link, the SU 3116 startstransmitting the access code 3142 back to the BS 3114 (step 3410). TheSU 3116 continuously increases the transmission power whileretransmitting the access code 3142 (step 3412) until it receives anacknowledgment from the BS 3114. The BS 3114 detects the access code3142 at the correct phase once the minimum power level for reception hasbeen achieved (step 3414). The BS 3114 subsequently transmits an accesscode detection acknowledgment signal (step 3416) to the SU 3116. Uponreceiving the acknowledgment, the SU ceases the transmission powerincrease (step 3418). With the power ramp-up completed, closed looppower control and call setup signaling is performed (step 3420) toestablish the two-way communication link.

Although this embodiment limits SU 3116 transmission power, acquisitionof the subscriber unit 3116 by the BS 3114 in this manner may lead tounnecessary power overshoot from the SU 3116, thereby reducing theperformance of the system 3110.

The transmission power output profile of the SU 3116 is shown in FIG.35. At t₀, the SU 3116 starts transmitting at the starting transmissionpower level P₀, which is a power level guaranteed to be less than thepower level required for detection by the BS 3114. The SU 3116continually increases the transmission power level until it receives thedetection indication from the BS 3114. For the BS 3114 to properlydetect the access code 3142 from the SU 3116 the access code 3142must: 1) be received at a sufficient power level; and 2) be detected atthe proper phase. Accordingly, referring to FIG. 35, although the accesscode 3142 is at a sufficient power level for detection by the BS 3114 att_(P), the BS 3114 must continue searching for the correct phase of theaccess code 3142 which occurs at t_(A).

Since the SU 3116 continues to increase the output transmission powerlevel until it receives the detection indication from the BS 3114, thetransmission power of the access code 3142 exceeds the power levelrequired for detection by the BS 3114. This causes unnecessaryinterference to all other SUs 3116. If the power overshoot is too large,the interference to other SUs 3116 may be so severe as to terminateongoing communications of other SUs 3116.

The rate that the SU 3116 increases transmission power to avoidovershoot may be reduced, however, this results in a longer call setuptime. Those of skill in the art would appreciate that adaptive ramp-uprates can also be used, yet these rates have shortcomings and will notappreciably eliminate power overshoot in all situations.

This embodiment of the present invention utilizes “short codes” and atwo-stage communication link establishment procedure to achieve fastpower ramp-up without large power overshoots. The spreading codetransmitted by the SU 3116 is much shorter than the rest of thespreading codes (hence the term short code), so that the number ofphases is limited and the BS 3114 can quickly search through the code.The short code used for this purpose carries no data.

The tasks performed by the BS 3114 and the SU 3116 to establish acommunication channel using short codes in accordance with thisembodiment of the present invention are shown in FIGS. 36A and 36B.During idle periods, the BS 3114 periodically and continuously transmitsthe pilot code to all SUs 3116 located within the operating range of theBS 3114 (step 3600). The BS 3114 also continuously searches for a shortcode transmitted by the SU 3116 (step 3602). The SU 3116 acquires thepilot code and synchronizes its transmit spreading code generator to thepilot code. The SU 3116 also periodically checks to ensure it issynchronized (step 3604). If synchronization is lost, the SU 3116reacquires the pilot signal transmitted by the BS (step 3606).

When a communication link is desired, the SU 3116 starts transmitting ashort code at the minimum power level P₀ (step 3608) and continuouslyincreases the transmission power level while retransmitting the shortcode (step 3610) until it receives an acknowledgment from the BS 3114that the short code has been detected by the BS 3114.

The access code in the preferred embodiment, as previously describedherein, is approximately 30 million chips in length. However, the shortcode is much smaller. The short code can be chosen to be any length thatis sufficiently short to permit quick detection. There is an advantagein choosing a short code length such that it divides the access codeperiod evenly. For the access code described herein, the short code ispreferably chosen to be 32, 64 or 128 chips in length. Alternatively,the short code may be as short as one symbol length, as will bedescribed in detail hereinafter.

Since the start of the short code and the start of the access code aresynchronized, once the BS 3114 acquires the short code, the BS 3114knows that the corresponding phase of the access code is an integermultiple of N chips from the phase of the short code where N is thelength of the short code. Accordingly, the BS 3114 does not have tosearch all possible phases corresponding to the maximum round trippropagation delay.

Using the short code, the correct phase for detection by the BS 3114occurs much more frequently. When the minimum power level for receptionhas been achieved, the short code is quickly detected (step 3612) andthe transmission power overshoot is limited. The transmission powerramp-up rate may be significantly increased without concern for a largepower overshoot. In the preferred embodiment of the present invention,the power ramp-up rate using the short code is 1 dB per millisecond.

The BS 3114 subsequently transmits a short code detection indicationsignal (step 3614) to the SU 3116 which enters the second stage of thepower ramp-up upon receiving this indication. In this stage, the SU 3116ceases transmitting the short code and starts continuously transmittinga periodic access code (step 3616). The SU 3116 continues to ramp-up itstransmission power while transmitting the access code, however theramp-up rate is now much lower than the previous ramp-up rate used withthe short code (step 3618). The ramp-up rate with the access code ispreferably 0.05 dB per millisecond. The slow ramp-up avoids losingsynchronization with the BS 3114 due to small changes in channelpropagation characteristics.

At this point, the BS 3114 has detected the short code at the properphase and power level (step 3612). The BS 3114 must now synchronize tothe access code which is the same length as all other spreading codesand much longer than the short code. Utilizing the short code, the BS3114 is able to detect the proper phase of the access code much morequickly. The BS 3114 begins searching for the proper phase of the accesscode (step 3620). However, since the start of the access code issynchronized with the start of the short code, the BS 3114 is onlyrequired to search every N chips; where N=the length of the short code.In summary, the BS 3114 quickly acquires the access code of the properphase and power level by: 1) detecting the short code; and 2)determining the proper phase of the access code by searching every Nchips of the access code from the beginning of the short code.

If the proper phase of the access code has not been detected aftersearching the number of phases in the maximum round trip delay the BS3114 restarts the search for the access code by searching every chipinstead of every N chips (step 3622). When the proper phase of theaccess code has been detected (step 3624) the BS 3114 transmits anaccess code detection acknowledgment (step 3626) to the SU 3116 whichceases the transmission power increase (step 3628) upon receiving thisacknowledgment. With the power ramp-up completed, closed loop powercontrol and call setup signaling is performed (step 3630) to establishthe two-way communication link.

Referring to FIG. 37, although the starting power level P₀ is the sameas in the prior embodiment, the SU 3116 may ramp-up the transmissionpower level at a much higher rate by using a short code. The short codeis quickly detected after the transmission power level surpasses theminimum detection level, thus minimizing the amount of transmissionpower overshoot.

Although the same short code may be reused by the SU 3116, in thepreferred embodiment of the present invention the short codes aredynamically selected and updated in accordance with the followingprocedure. Referring to FIG. 38, the period of the short code is equalto one symbol length and the start of each period is aligned with asymbol boundary. The short codes are generated from a regular lengthspreading code. A symbol length portion from the beginning of thespreading code is stored and used as the short code for the next 3milliseconds. Every 3 milliseconds, a new symbol length portion of thespreading code replaces the old short code. Since the spreading codeperiod is an integer multiple of 3 milliseconds, the same short codesare repeated once every period of the spreading code.

A block diagram of the BS 3114 is shown in FIG. 39. Briefly described,the BS 3114 comprises a receiver section 3150, a transmitter section3152 and a diplexer 3154. An RF receiver 3156 receives and down-convertsthe RF signal received from the diplexer 3154. The receive spreadingcode generator 3158 outputs a spreading code to both the data receiver3160 and the code detector 3162. In the data receiver 3160, thespreading code is correlated with the baseband signal to extract thedata signal which is forwarded for further processing. The receivedbaseband signal is also forwarded to the code detector 3162 whichdetects the access code or the short code from the SU 3116 and adjuststhe timing of the spreading code generator 3158 to establish acommunication channel 3118.

In the transmitter section 3152 of the BS 3114, the transmit spreadingcode generator 3164 outputs a spreading code to the data transmitter3166 and the pilot code transmitter 3168. The pilot code transmitter3168 continuously transmits the periodic pilot code. The datatransmitter 3166 transmits the short code detect indication and accesscode detect acknowledgment after the code detector 3162 has detected theshort code or the access code respectively. The data transmitter alsosends other message and data signals. The signals from the datatransmitter 3166 and the pilot code transmitter 3168 are combined andup-converted by the RF transmitter 3170 for transmission to the SUs3116.

A block diagram of the SU 3116 is shown in FIG. 40. Briefly described,the SU 3116 comprises a receiver section 3172, a transmitter section3174 and a diplexer 3184. An RF receiver 3176 receives and down-convertsthe RF signal received from the diplexer 3184. A pilot code detector3180 correlates the spreading code with the baseband signal to acquirethe pilot code transmitted by the BS 3114. In this manner, the pilotcode detector 3180 maintains synchronization with the pilot code. Thereceiver spreading code generator 3182 generates and outputs a spreadingcode to the data receiver 3178 and the pilot code detector 3180. Thedata receiver 3178 correlates the spreading code with the basebandsignal to process the short code detect indication and the access codedetect acknowledgment transmitted by the BS 3114.

The transmitter section 3174 comprises a spreading code generator 3186which generates and outputs spreading codes to a data transmitter 3188and a short code and access code transmitter 3190. The short code andaccess code transmitter 3190 transmits these codes at different stagesof the power ramp-up procedure as hereinbefore described. The signalsoutput by the data transmitter 3188 and the short code and access codetransmitter 3190 are combined and up-converted by the RF transmitter3192 for transmission to the BS 3114. The timing of the receiverspreading code generator 3182 is adjusted by the pilot code detector3180 through the acquisition process. The receiver and transmitterspreading code generators 3182, 3186 are also synchronized.

An overview of the ramp-up procedure in accordance with this embodimentof the invention is summarized in FIGS. 41A and 41B. The BS 3114transmits a pilot code while searching for the short code (step 3200).The SU 3116 acquires the pilot code transmitted from the BS 3114 (step3202), starts transmitting a short code starting at a minimum powerlevel P₀ which is guaranteed to be less than the required power, andquickly increases transmission power (step 3204). Once the receivedpower level at the BS 3114 reaches the minimum level needed fordetection of the short code (step 3206) the BS 3114 acquires the correctphase of the short code, transmits an indication of this detection, andbegins searching for the access code (step 3208). Upon receiving thedetection indication, the SU 3116 ceases transmitting the short code andstarts transmitting an access code. The SU 3116 initiates a slow ramp-upof transmit power while sending the access code (step 3210). The BS 3114searches for the correct phase of the access code by searching only onephase out of each short code length portion of the access code (step3212). If the BS 3114 searches the phases of the access code up to themaximum round trip delay and has not detected the correct phase, thesearch is repeated by searching every phase (step 3214). Upon detectionof the correct phase of the access code by the BS 3114, the BS 3114sends an acknowledgment to the SU 3116 (step 3216). Reception of theacknowledgment by the SU 3116 concludes the ramp-up process. A closedloop power control is established, and the SU 3116 continues the callsetup process by sending related call setup messages (step 3218).

XXXXIII. Virtual Locating of a Fixed SU to Reduce Re-Acquisition Time

A typical CDMA communication system is shown in FIG. 42. The systemcomprises a BS and a plurality of fixed subscriber units SU₁-SU₇ locatedat various distances from the BS. The BS constantly transmits a forwardpilot signal. The SUs maintain epoch alignment between the forward pilotsignal and their internal PN code generator such that all signalstransmitted from an SU are at the same PN code phase at which theforward pilot is received. The BS receives signals from SUs with a codephase difference between its forward pilot signal and the receivedsignal corresponding to the two-way signal propagation delay between theBS and the SU.

For the BS to detect a signal, it must align the phase of its receive PNcode generator to the phase of the received signal, thus “acquiring” thesignal. The BS can receive an access signal with any code phasedifference within the range of the cell. Therefore, the BS must test allcode phases associated with the range of possible propagation delays ofthe cell to acquire the access signal.

Once a communication channel is established between the BS and the SU,the transmission power of the SU is controlled by a closed loop APCalgorithm which prevents the power from each SU from excessivelyinterfering with other SUs. During channel establishment, before theclosed loop power control begins, an SU's transmission power is kept toa minimum by ramping-up from a low level and establishing the channelwithout the SU significantly overshooting (on the order of less than 3dB) the minimum power necessary to operate the channel.

To establish a channel, each SU transmits an access signal for detectionby the BS. The BS acquires the access signal and transmits aconfirmation signal to each SU. The time required for the BS to acquirethe access signal contributes directly to the time elapsed between a SUgoing “off hook”, establishing a communication channel, connecting tothe LE and receiving a dial tone. It is desirable to receive a dial tonewithin 150 msec of detection of “off hook”.

The time distribution of acquisition opportunities is shown in FIG. 43for a typical prior art SU located 20 km from a BS in a 30 km cell. Fora BS which tests 8 code phases simultaneously at a PN rate of 12.48 MHzand a symbol rate of 64,000 symbols per second and takes an average of7.5 symbol periods to accept or reject a particular group of codephases, the average time to test all code phase delays within the cellis approximately 37 msec, and any one SU can only be detected during anapproximately 100 μsec window during that period. Assuming that theselection of initial SU transmission power level is 15-20 dB below theproper level and a slow ramp-up rate of between 0.05 and 0.1 dB/msec, itcould take 4-5 such 37 msec time periods, (or an average ofapproximately 200 msec,) for the BS to acquire a SU. This introduces anunacceptable delay in the channel establishment process which should beless than 150 msec. Accordingly, there is a need to reduce the amount oftime required for a BS to acquire an SU.

The present invention includes a method of reducing the re-acquisitiontime of a fixed SU by a BS in a CDMA communication system by utilizingvirtual locating of the SU. A BS acquires SUs by searching only thosecode phases concomitant with the largest propagation delay possible inthe cell, as if all SUs were located at the periphery of the cell. An SUwhich has never been acquired by the BS varies the delay between the PNcode phase of its received and transmitted signals over the range ofpossible delays in a cell and slowly ramps-up its transmission poweruntil it is acquired by the BS. Upon initial acquisition by the BS theSU ceases ramping-up its power and varying the delay and internallystores the final value of the delay in memory. For subsequentre-acquisition, the SU adds the delay value between the PN code phase ofits received and transmitted signals, making the subscriber virtuallyappear to be at the periphery of the cell. This permits a quick ramp-upof transmission power by the SU and reduced acquisition time by the BS.

Referring to FIG. 44, the propagation of certain signals in theestablishment of a communication channel 4018 between a BS 4014 and aplurality of SUs 4016 is shown. The forward pilot signal 4020 istransmitted from the BS 4014 at time t0, and is received by a SU 4016after a propagation delay Δt. To be acquired by the BS 4014 the SU 4016transmits an access signal 4022 which is received by the BS 4014 after afurther propagation delay of Δt. Accordingly, the round trip propagationdelay is 2Δt. The access signal 4022 is transmitted epoch aligned to theforward pilot signal 4020, which means that the code phase of the accesssignal 4022 when transmitted is identical to the code phase of thereceived forward pilot signal 4020.

The round trip propagation delay depends upon the location of an SU 4016with respect to the BS 4014. Communication signals transmitted between aSU 4016 located closer to the BS 4014 will experience a shorterpropagation delay than an SU 4016 located further from the BS 4014.Since the BS 4014 must be able to acquire SUs 4016 located at anyposition within the cell 4030, the BS 4014 must search all code phasesof the access signal corresponding to the entire range of propagationdelays of the cell 4030.

It should be apparent to those of skill in the art that theestablishment of a communication channel between a BS 4014 and an SU4016 is a complex procedure involving many tasks performed by the BS4014 and the SU 4016 which are outside the scope of the presentinvention. The present invention is directed to decreasing thereacquisition time of a fixed SU 4016 by a BS 4014 during there-establishment of a communication channel.

Referring to FIG. 45, the tasks associated with initial acquisition ofan SU 4016 by a BS 4014 in accordance with the preferred embodiment ofthe present invention are shown. When an SU 4016 desires theestablishment of a channel 4018 with a BS 4014 with which it has neverestablished a channel, the SU 4016 has no knowledge of the two-waypropagation delay. Accordingly, the SU 4016 enters the initialacquisition channel establishment process.

The SU 4016 selects a low initial power level and zero code phase delay,(epoch aligning the code phase of the transmitted access signal 4022 tothe code phase of the received forward pilot signal 4020), and commencestransmitting the access signal 4022 while slowly (0.05-0.1 dB/msec)ramping-up transmission power (step 4100). While the SU 4016 is awaitingreceipt of the acknowledgement signal from the BS 4014, it varies thecode phase delay in predetermined steps from zero to the delaycorresponding to the periphery of the cell 4030, (the maximum code phasedelay), allowing sufficient time between steps for the BS 4014 to detectthe access signal 4022 (step 4102). If the SU 4016 reaches the codephase delay corresponding to the periphery of the cell 4030, it repeatsthe process of varying the code phase delay while continuing the slowpower ramp-up (step 4102).

In order to acquire SUs 4016 desiring access, the BS 4014 continuouslytransmits a forward pilot signal 4020 and attempts to detect the accesssignals 4022 from SUs 4016 (step 4104). Rather than test for accesssignals 4022 at all code phase delays within the cell 4030 as withcurrent systems, the BS 4014 need only test code phase delays centeredabout the periphery of the cell 4030.

The BS 4014 detects the access signal 4022 (step 4106) when the SU 4016begins transmitting with sufficient power at the code phase delay whichmakes the SU 4016 appear to be at the periphery of the cell 4030,thereby “virtually” locating the SU 4016 at the periphery of the cell4030. The BS 4014 then transmits an acknowledgement to the SU 4016 whichconfirms that the access signal 4022 has been received (step 4108) andcontinues with the channel establishment process (step 4110).

Once the SU 4016 receives the acknowledgement signal (step 4112), itceases the ramp-up of transmission power, ceases varying the code phasedelay (step 4114) and records the value of the code phase delay forsubsequent re-acquisitions (step 4116). The SU 4016 then continues thechannel establishment process including closed-loop power transmissioncontrol (step 4118).

On subsequent re-acquisitions when an SU 4016 desires the establishmentof a channel 4018 with a BS 4014, the SU 4016 enters the re-acquisitionchannel establishment process shown in FIG. 46. The SU 4016 selects alow initial power level and the code phase delay recorded during theinitial acquisition process, (shown in FIG. 45), and commencescontinuously transmitting the access signal 4022 while quickly (1dB/msec) ramping-up transmission power (step 4600). While the SU 4016 isawaiting receipt of the acknowledgement signal from the BS 4014, itslightly varies the code phase delay of the access signal 4022 about therecorded code phase delay, allowing sufficient time for the BS 4014 todetect the access signal 4022 before changing the delay (step 4602). TheBS 4014 as in FIG. 45, transmits a forward pilot signal 4020 and testsonly the code phase delays at the periphery of the cell 4030 inattempting to acquire the SUs 4016 within its operating range (step4604). The BS 4014 detects the access signal 4022 when the SU 4016transmits with sufficient power at the code phase delay which makes theSU 4016 appear to be at the periphery of the cell 4030 (step 4606). TheBS 4014 transmits an acknowledgement to the SU 4016 which confirms thatthe access signal 4022 has been received (step 4608) and continues withthe channel establishment process (step 4610).

When the SU 4016 receives the acknowledgement signal (step 4612) itceases power ramp-up, ceases varying the code phase delay (step 4614)and records the present value of the code phase delay for subsequentre-acquisitions (step 4616). This code phase delay may be slightlydifferent from the code phase delay initially used when starting there-acquisitions process. The SU 4016 then continues the channelestablishment process at the present power level (step 4618). If an SU4016 has not received an acknowledgement signal from the BS 4014 after apredetermined time, the SU 4016 reverts to the initial acquisitionprocess described in FIG. 45.

The effect of introducing a code phase delay in the Tx 4020 and Rx 4022communications between the BS 4014 and an SU 4016 will be explained withreference to FIGS. 47 and 48. Referring to FIG. 47A, a BS 4160communicates with two SUs 4162, 4164. The first SU 4162 is located 30 kmfrom the BS 4160 at the maximum operating range. The second SU 4164 islocated 15 km from the BS 4160. The propagation delay of Tx and Rxcommunications between the first SU 4162 and the BS 4160 will be twicethat of communications between the second SU 4164 and the BS 4160.

Referring to FIG. 48, after an added delay value 4166 is introduced intothe Tx PN generator of the second SU 4164 the propagation delay ofcommunications between the first SU 4162 and the BS 4160 will be thesame as the propagation delay of communications between the second SU4164 and the BS 4160. Viewed from the BS 4160, it appears as though thesecond SU 4164 is located at the virtual range 4164′.

Referring to FIG. 49, it can be seen that when a plurality of SUs,SU₁-SU₇, are virtually relocated to the virtual range 4175, the BS mustonly test the code phase delays centered about the virtual range 4175.

Utilizing the present invention, an SU 4016 which has achieved asufficient power level will be acquired by the BS 4014 in approximately2 msec. Due to the shorter acquisition time, the SU 4016 can ramp-up ata much faster rate, (on the order of 1 dB/msec), without significantlyovershooting the desired power level. Assuming the same 20 dB powerback-off, it would take the SU 4016 approximately 20 msec to reach thesufficient power level for detection by the BS 4014. Accordingly, theentire duration of the re-acquisition process of the present inventionis approximately 22 msec, which is an order of magnitude reduction fromprior art reacquisition methods.

An SU 4200 made in accordance with one embodiment of the presentinvention is shown in FIG. 50. The SU 4200 includes a receiver section4202 and a transmitter section 4204. An antenna 4206 receives a signalfrom the BS 4014, which is filtered by a band-pass filter 4208 having abandwidth equal to twice the chip rate and a center frequency equal tothe center frequency of the spread spectrum system's bandwidth. Theoutput of the filter 4208 is down-converted by a mixer 4210 to abaseband signal using a constant frequency (Fc) local oscillator. Theoutput of the mixer 4210 is then spread spectrum decoded by applying aPN sequence to a mixer 4212 within the PN Rx generator 4214. The outputof the mixer 4212 is applied to a low pass filter 4216 having a cutofffrequency at the data rate (Fb) of the PCM data sequence. The output ofthe filter 4216 is input to a codec 4218 which interfaces with thecommunicating entity 4220.

A baseband signal from the communicating entity 4220 is pulse codemodulated by the codec 4218. Preferably, a 32 kilobit per secondadaptive pulse code modulation (ADPCM) is used. The PCM signal isapplied to a mixer 4222 within a PN Tx generator 4224. The mixer 4222multiplies the PCM data signal with the PN sequence. The output of themixer 4222 is applied to low-pass filter 4226 whose cutoff frequency isequal to the system chip rate. The output of the filter 4226 is thenapplied to a mixer 4228 and suitably up-converted, as determined by thecarrier frequency Fc applied to the other terminal. The up-convertedsignal is then passed through a band-pass filter 4230 and to a broadbandRF amplifier 4232 which drives an antenna 4234.

The microprocessor 4236 controls the acquisition process as well as theRx and Tx PN generators 4214, 4224. The microprocessor 4236 controls thecode phase delay added to the Rx and Tx PN generators 4214, 4224 toacquire the forward pilot signal 4020, and for the SU 4200 to beacquired by the BS 4014, and records the code phase difference betweenthese PN generators. For re-acquisition the microprocessor 4236 adds therecorded delay to the Tx PN generator 4224.

The BS 4014 uses a configuration similar to the SU 4016 to detect PNcoded signals from the SU 4200. The microprocessor (not shown) in the BS4014 controls the Rx PN generator in a similar manner to make the codephase difference between Rx PN generator and the Tx PN generatorequivalent to the two-way propagation delay of the SU's 4016 virtuallocation. Once the BS 4014 acquires the access signal 4022 from the SU4016, all other signals from the SU 4016 to the BS 4014 (traffic, pilot,etc.) use the same code phase delay determined during the acquisitionprocess.

It should be noted that although the invention has been described hereinas the virtual locating of SUs 4016 at the periphery of the cell 4030the virtual location can be at any fixed distance from the BS 4014.

Referring to FIG. 51, the tasks associated with initial acquisition of a“never-acquired” SU 4016 by a BS 4014 in accordance with an alternativeembodiment of the present invention are shown. The SU 4016 continuouslytransmits an epoch aligned access signal 4022 to the BS 4014 (step 4300)when the establishment of a channel 4018 is desired. While the SU 4016is awaiting the receipt of a confirmation signal from the BS 4014, itcontinuously increases the transmission power as it continuestransmission of the access signal 4022 (step 4302).

To detect SUs which have never been acquired, the BS 4014 transmits aforward pilot signal 4020 and sweeps the cell by searching all codephases corresponding to the entire range of propagation delays of thecell (step 4304) and detects the epoch aligned access signal 4022 sentfrom the SU 4016 after the transmission has achieved sufficient powerfor detection (step 4306). The BS 4014 transmits an acknowledgement tothe SU 4016 (step 4308) which confirms that the access signal 4022 hasbeen received. The SU 4016 receives the acknowledgment signal (step4310) and ceases the increase in transmission power (step 4312).

The BS 4014 determines the desired code phase delay of the SU 4016 bynoting the difference between the Tx and Rx PN generators 4224, 4214after acquiring the SU 4016. The desired code phase delay value is sentto the SU 4016 (step 4316) as an OA&M message, which receives and storesthe value (step 4318) for use during re-acquisition, and continues withthe channel establishment process (steps 4322 and 4324).

Referring to FIG. 52, an alternative method of fast reacquisition inaccordance with the present invention is shown. When a communicationchannel must be reestablished between the SU 4016 and the BS 4014, theSU 4016 transmits the access signal 4022 with the desired code phasedelay as in the preferred embodiment.

With all of the previously acquired SUs 4016 at the same virtual range,the BS 4014 need only search the code phase delays centered about theperiphery of the cell to acquire the access signals 4022 of such SUs4016 (step 4330). Thus, an SU 4016 may ramp-up power rapidly to exploitthe more frequent acquisition opportunities. The SU 4016 implements thedelay the same way as in the preferred embodiment. The BS 4014subsequently detects the SU 4016 at the periphery of the cell (step4336), sends an acknowledgment signal to the SU (step 4337) andrecalculates the desired code phase delay value, if necessary.Recalculation (step 4338) compensates for propagation path changes,oscillator drift and other communication variables. The BS 4014 sendsthe updated desired code phase delay value to the SU 4016 (step 4340)which receives and stores the updated value (step 4342). The SU 4016 andthe BS 4014 then continue the channel establishment processcommunications (steps 4344 and 4346).

Note that this embodiment requires the BS to search both the code phasedelays centered on the periphery of the cell to re-acquire previouslyacquired SUs and the code phase delays for the entire cell to acquiredSUs which have never been acquired.

Referring to FIG. 53, the tasks associated with initial acquisition of anever-acquired SU 4016 by a BS 4014 in accordance with a secondalternative embodiment of the present invention are shown. In theembodiment shown in FIG. 51, when a never-acquired SU 4016 is acquired,the access signal 4020 remains epoch aligned to the forward pilot signal4020. In this embodiment, the BS 4014 and SU 4016 change the code phasealignment of the access signal 4022 from epoch aligned to delayed, (bythe code phase delay), to make the SU 4016 appear at the periphery ofthe cell. This change is performed at a designated time.

Steps 4400 through 4418 are the same as the corresponding steps 4300through 4318 shown in FIG. 51. However, after the BS 4014 sends thedesired delay value to the SU 4016 (step 4416) the BS 4014 sends amessage to the SU 4016 to switch to the desired delay value at a timereferenced to a sub-epoch of the forward pilot signal 4020 (step 4420).The SU 4016 receives this message (step 4422), and both units 4014, 4016wait until the switchover time is reached (steps 4424, 4430). At thattime, the BS 4014 adds the desired delay value to its Rx PN operator(step 4432) and the SU 4016 adds the same desired delay value to its TxPN generator (step 4426). The SU 4016 and the BS 4014 then continue thechannel establishment process communication (step 4428, 4434).

XXXXIV. Parallel Packetized Intermodule Arbitrated High Speed Controland Data Bus

For communication within a digital device, such as between a CPU(central processing unit), memory, peripherals, I/O (input/output)devices, or other data processors, a communication bus may be employed.As shown in FIG. 54, a communication bus is a set of shared electricalconductors for the exchange of digital words. In this manner,communication between devices is simplified, thereby obviating separateinterconnections.

A communication bus typically contains a set of data lines, addresslines for determining which device should transmit or receive, andcontrol and strobe lines that specify the type of command beingexecuted. The address and strobe lines communicate one-way from the CPU.Typically, all data lines are bidirectional. Data lines are asserted bythe CPU during the write instruction, and by the peripheral deviceduring read. Both the CPU and peripheral device use three-state driversfor the data lines.

In a computer system where several data processing devices exchange dataon a shared data bus, the two normal states of high and low voltage(representing the binary 1's and 0's) may be implemented by an activevoltage pullup. However, when several processing modules are exchangingdata on a data bus, a third output state, open circuit, must be added sothat another device located on the bus can drive the same line.

Three-state or open-collector drivers are used so that devices connectedto the bus can disable their bus drivers, since only one device isasserting data onto the bus at a given time. Each bus system has adefined protocol for determining which device asserts data. A bus systemis designed so that, at most, one device has its drivers enabled at onetime with all other devices disabled (third state). A device knows toassert data onto the bus by recognizing its own address on the controllines. The device looks at the control lines and asserts data when itsees its particular address on the address lines and a read pulse.However, there must be some external logic ensuring that the three-statedevices sharing the same lines do not talk at the same time or buscontention will result.

Bus control logic or a “bus master” executes code for the protocol usedto arbitrate control of the bus. The bus master may be part of a CPU orfunction independently. More importantly, control of the bus may begranted to another device. More complex bus systems permit other deviceslocated on the bus to master the bus.

Data processing systems have processors which execute programmedinstructions stored in a plurality of memory locations. As shown in FIG.54, the processed data is transferred in and out of the system onto thebus, by using I/O devices interconnecting with other digital devices. Abus protocol, or handshaking rules delineate a predetermined series ofsteps to permit data exchange between the devices.

To move data on a shared bus, the data, recipient and moment oftransmission must be specified. Therefore, data, address and a strobeline must be specified. There are as many data lines as there are bitsin a word to enable a whole word to be transferred simultaneously. Datatransfer is synchronized by pulses on additional strobe bus lines. Thenumber of address lines determines the number of addressable devices.

Communication buses are either synchronous or asynchronous. In asynchronous bus, data is asserted onto or retrieved from the bussynchronously with strobing signals generated by the CPU or elsewhere inthe system. However, the device sending the data does not know if thedata was received. In an asynchronous bus, although handshaking betweencommunicating devices assures the sending device that the data wasreceived, the hardware and signaling complexity is increased.

In most high-speed, computationally intensive multichannel dataprocessing applications, digital data must be moved very rapidly to orfrom another processing device. The transfer of data is performedbetween memory and a peripheral device via the bus without programintervention. This is also known as direct memory access (DMA). In DMAtransfers, the device requests access to the bus via special bus requestlines and the bus master arbitrates how the data is moved, (either inbytes, blocks or packets), prior to releasing the bus to the CPU.

A number of different types of bus communication systems and protocolsare currently in use today to perform data transfer. As shown in thetable of FIG. 55, various methods have been devised to manipulate databetween processing devices. Data communication buses having powerfulsynchronous/high-level data link control SDLC/HDLC protocols exist,along with standardized parallel transmission such as small computersystem interface (SCSI) and carrier-sensemultiple-access/collision-detection (CSMA/CD)(Ethernet) networks.However, in specialized, high-speed applications, a simplified datacommunication bus is desired.

The present invention includes a parallel packetized intermodulearbitrated high speed control data bus system which allows high speedcommunications between microprocessor modules in a more complex digitalprocessing environment. The system features a simplified hardwarearchitecture featuring fast first-in/first-out (FIFO) queuing operatingat 12.5 MHz, TTL CMOS compatible level clocking signals, single busmaster arbitration, synchronous clocking, DMA, and unique moduleaddressing for multiprocessor systems. The present invention includes aparallel data bus with sharing bus masters residing on each processingmodule decreeing the communication and data transfer protocols.

The high-speed intermodule communication bus (HSB) is used forcommunication between various microprocessor modules. The data bus issynchronous and completely bidirectional. Each processing module thatcommunicates on the bus will have the described bus controlarchitecture. The HSB comprises eight shared parallel data lines for theexchange of digital data and two additional lines for arbitration andclock signals. No explicit bus request or grant signals are required.The HSB can also be configured as a semi-redundant system, duplicatingdata lines while maintaining a single component level. The bus is drivenby three-state gates with resistor pullups serving as terminators tominimize signal reflections.

To move data on the HSB, each processing module must specify the data,the recipient, and the moment when the data is valid. Only one messagesource, known as the bus master, is allowed to drive the bus at anygiven time. Since the data flow is bidirectional, the bus arbitrationscheme establishes a protocol of rules to prevent collisions on the datalines when a given processing module microprocessor is executinginstructions. The arbitration method depends on the detection ofcollisions present only on the arbitration bus and uses state machineson each data processing module to determine bus status. Additionally,the arbitration method is not daisy chained, allowing greater systemflexibility.

The state machines located on each processing module are the controllinginterface between the microprocessor used within a given processingmodule and the HSB. The circuitry required for the interface iscomprised of a transmit FIFO, receive FIFO, miscellaneousdirectional/bidirectional signal buffers and the software code for thestate machines executed in an erasable programmable logic device (EPLD).

The HSB 5020 of the present invention is shown in simplified form inFIG. 56. The preferred embodiment comprises a bus controller 5022, atransmit FIFO 5024, a receive FIFO 5026, an eight bit parallel data bus5028 and a serial arbitration bus 5050. The ends of the bus 5028 areterminated with a plurality of resistive dividers to minimize signalreflections. An internal 8 bit address and data bus 5030 couples thetransmit 5024 and receive 5026 FIFOs and bus controller 5022 to a CPU5032 and DMA controller 5033 located on a given processor module 5034.The internal address and data bus 5030 also permits communicationbetween the CPU 5032 and bus controller 5022 and various memory elementssuch as PROM 5036, SRAM 5038, and DRAM 5040 required to support theapplications of the data processing module 5034.

The HSB 5020 is a packetized message transfer bus system. Variousprocessor modules 5034 can communicate data, control and status messagesvia the present invention.

The HSB 5020 provides high speed service for a plurality of processormodules 5034 with minimum delay. The message transfer time betweenmodules is kept short along with the overhead of accessing the data bus5028 and queuing each message. These requirements are achieved by usinga moderately high clock rate and a parallel data bus 5028 architecture.Transmit 5024 and receive 5026 FIFOs are used to simplify and speed upthe interface between a processor module 5034 CPU 5032 and the data bus5028.

Referring to FIGS. 57A-57E, a common clock signal (HSB_CLK) 5042comprising a TTL compatible CMOS level signal with a frequency nominally12.5 MHz and a duty cycle of approximately 50% synchronizes all HSB 5020components and executions. The clock 5042 pulse may originate in anypart of the complete digital system and its origination is beyond thescope of this disclosure.

The parallel data bus 5028 (HSB_DAT) lines 0-7, provides 8 bidirectionalTTL compatible CMOS level signals. Only one message source, the buscontroller or master 5022, is allowed to drive the bus 5028 at any onetime. A bus arbitration scheme determines which out of a plurality ofprocessing modules may become bus master and when.

The relationship of the data 5028 and control signal transitions to theclock 5042 edges are important to recovering the data reliably at areceiving module. Data is clocked out from a transmitting module 5034onto the data bus 5028 with the negative or trailing edge of the clocksignal 5042. The data is then clocked on the positive or leading edge ofthe clock signal 5042 at an addressed receiving module. This featureprovides a sufficient setup and hold time of approximately 40 ns withoutviolating the minimum setup time for octal register 5060.

Before data can be transmitted on the data bus 5028, the bus controller5022 must obtain permission from the arbitration bus 5050 to prevent apossible data collision. The message source must win an arbitration froma potential multiplicity of processor module 5034 access requests. Thewinner is granted temporary bus mastership for sending a single message.After the transfer of data is complete, bus mastership is relinquished,thereby permitting bus 5028 access by other processor modules 5034.

No explicit bus request and grant signals are required with the serialarbitration method of the present invention. The preferred methodeliminates complex signaling and signal lines, along with the requisitecentralized priority encoder and usual granting mechanism. Thearbitration method is not daisy chained so that any processor modulelocation on the bus 5028 may be empty or occupied without requiring achange to address wiring.

In the present invention, the open-collector arbitration bus 5050permits multiple processing modules 5034 to compete for control of thedata bus 5028. Since no processing module 5034 in the digital systemknows a priori if another processing module has accessed the arbitrationbus 5050, modules within the HSB system may drive high and low levellogic signals on the HSB simultaneously, causing arbitration collisions.The collisions occur without harm to the driving circuit elements.However, the collisions provide a method of determining bus activity.

The arbitration bus 5050 includes pullup resistors connected to aregulated voltage source to provide a logic 1 level. The arbitration busdriver 5052 connects the arbitration bus 5050 to ground to drive a logic0 level. This results in a logic 1 only when no other processing module5034 drives a logic 0. The arbitration bus 5050 will be low if anyprocessing module 5034 arbitration bus 5050 driver 5052 asserts a logic0.

As known to those familiar with the art, the connection is called“wired-OR” since it behaves like a large NOR gate with the line goinglow if any device drives high (DeMorgan's theorem). An active lowreceiver inverts a logic 0 level, producing an equivalent OR gate. Usingpositive-true logic conventions yields a “wired-AND,” using negativelogic yields a “wired-OR.” This is used to indicate if at least onedevice is driving the arbitration bus 5050 and does not requireadditional logic. Therefore, if a processing module 5034 asserts a logic1 on the arbitration bus 5050 and monitors a logic 0, via buffer 5053 onmonitor line 5055 (BUS_ACT_N), the processing module 5034 bus controller5022 determines that a collision has occurred and that it has lost thearbitration for access.

The arbitration method depends on the detection of collisions and usesstate machines 5046 and 5048 within the bus controller 5022 on eachprocessing module 5034 to determine arbitration bus 5050 status asarbitration proceeds. All transitions on the arbitration bus 5050 aresynchronized to the bus clock 5042. Each processor module 5034 has aunique programmed binary address to present to the arbitration bus 5050.The device address in the current embodiment is six bits, therebyyielding 63 unique processing module 5034 identifications.

Each processing module 5034 bus controller 5022 located on the HSB 5020monitors, (via a buffer 5053), and interrogates, (via a buffer 5052),the arbitration bus (HSBI_ARB1_N) 5050. Six or more high level signalsclocked indicate that the bus is not busy. If a processing module 5034desires to send a message, it begins arbitration by serially shiftingout its own unique six bit address onto the arbitration bus 5050starting with the most significant bit. Collisions will occur on thearbitration bus 5050 bit by bit as each bit of the six bit address isshifted out and examined. The first detected collision drops theprocessing module 5034 wishing to gain access out of the arbitration. Ifthe transmit state machine 5046 of the sending module 5034 detects acollision it will cease driving the arbitration bus 5050, otherwise itproceeds to shift out the entire six bit address. Control of the databus 5028 is achieved if the entire address shifts out successfully withno errors.

A priority scheme results since logic 0's pull the arbitration bus 5050low. Therefore, a processor module 5034 serially shifting a string oflogic 0's that constitute its address will not recognize a collisionuntil a logic 1 is shifted. Addresses having leading zeroes effectivelyhave priority when arbitrating for the bus 5050. As long as bus 5028traffic is not heavy, this effect will not be significant.

In an alternative embodiment, measures can be taken to add equitybetween processor modules 5034 if required. This can be done by alteringmodule arbitration ID's or the waiting period between messages.

Once a processor module 5034 assumes bus mastership it is free to senddata on the data bus 5028. The bus controller 5022 enables its octal bustransceiver (driver) 5060 and transmits at the clock 5042 rate. Themaximum allowed message length is 512 bytes. Typically, messages will be256 bytes or shorter. After a successful arbitration, the arbitrationbus 5050 is held low by the transmitting processor module 5034 duringthis period as an indication of a busy arbitration bus 5050.

Once the data transfer is complete, the bus controller 5022 disables itsoctal bus transceiver (drivers) 5060 via line 5054 (HSB_A_EN_N) andreleases the arbitration bus 5050 to high. Another arbitration anywherein the system may then take place.

An alternative embodiment allows bus 5028 arbitration to take placesimultaneous with data transfer improving on data throughput throughoutthe digital system. In the preferred embodiment, the delay is consideredinsignificant obviating the added complexity.

The bus controller 5022 is required to control the interface between theprocessing module 5034 microprocessor 5032 and the HSB 5020 and betweenthe HSB and the bus (data bus 5028 and arbitration bus 5050) signals. Inthe preferred embodiment the bus controller 5022 is an Altera 7000series erasable programmable logic device (EPLD). The 8 bit internaldata bus 5030 interfaces the bus controller 5022 with the processormodule 5034 CPU 5032. The processor module 5034 CPU 5032 will read andwrite directly to the bus controller 5022 internal registers via theinternal data bus 5030. The bus controller 5022 monitors the arbitrationbus 5050 for bus status. This is necessary to gain control for outgoingmessages and to listen and recognize its address to receive incomingmessages. The bus controller 5022 monitors and controls the data FIFO's5024 and 5025, DMA controller 5033 and bus buffer enable 5054.

The components used in the preferred embodiment are shown in Table 15.

TABLE 15 MANU- QTY FACTURER PART NUMBER DESCRIPTION ELEMENT 1 IDTIDT7202LA-50J 1Kx9 Receive 5024 or FIFO Samsung KM75C02AJ50 1 IDTIDT7204LA-50J 4Kx9 Transmit 5026 or FIFO Samsung KM75C04AJ50 1 TISN74ABT125 Quad tristate 5058 or driver TI SN74BCT125 3 TI SN74ABT245TTL Octal 5060 or Buffers TI SN74BCT245 1 Altera 7128E erasable 5022programmable logic device

Address decoding and DMA gating are required and are performed in thebus controller 5022. The bus controller 5022 also contains a number ofinternal registers that can be read or written to. The CPU 5032communicates with and instructs the bus controller 5022 over the 8 bitinternal data bus 5030.

Loading the transmit FIFO 5024 is handled by the bus controller 5028,DMA and address decoding circuits contained within the bus controller5022. Gaining access to the bus 5028 and unloading the FIFO 5024 ishandled by the transmit state machine.

On power up, the bus controller 5022 receives a hardware reset 56. Theapplication software running on the processor module 5034 CPU 5032 hasthe option of resetting the bus controller 5022 via a write strobe ifthe application requires a module reset. After a reset, the buscontroller 5022 monitors the arbitration bus 5050 on line 5055 todetermine bus activity and to sync with the data bus 5028.

After a period of inactivity the bus controller 5022 knows that the bus5028 is between messages and not busy. A processor module 5034 can thenrequest control of the bus via arbitration. If no messages are to besent, the bus controller 5022 continues to monitor the arbitration bus5050.

The processor module CPU 5032 writes messages into the transmit FIFO5024 at approximately 20 Mbps. The DMA controller, a Motorola 68360 5033running at 25 MHz will be able to DMA the transmit FIFO 5024 atapproximately 12.5 Mbps. Since only one message is allowed in thetransmit FIFO 5024 at any one time, the CPU 5032 must buffer additionaltransmit messages in its own RAM 5040. Since the maximum allowablemessage length is 512 bytes with anticipated messages averaging 256bytes, a FIFO length of 1 Kb is guaranteed not to overflow. Once amessage has been successfully sent, the transmit FIFO 5024 flags emptyand the next message can be loaded.

A typical 256 byte message sent by a processing module 5034 CPU 5032 at12.5 MBps will take less than 21 μsec from RAM 5040 to transmit FIFO5024. Bus arbitration should occupy not more than 1 μsec if the bus isnot busy. Total elapsed time from the loading of one transmit message tothe next is approximately 43 to 64 μsec. Since not many messages canqueue during this period, circular RAM buffers are not required.

As shown in FIGS. 58 and 60, during DMA transfers, the DMA controller5033 disables the processor module 5034 CPU 5032 and assumes control ofthe internal data bus 5030. The DMA transfer is brought about by theprocessor module 5034 or by a request from another processor module5134. The other processor 5134 successfully arbitrates control of thedata bus 5028 and signals the processor module CPU 5032. The CPU 5032gives permission and releases control of bus 5030. The processor moduleCPU 5032 signals the DMA controller 5033 to initiate a data transfer.The DMA controller 5033 generates the necessary addresses and tracks thenumber of bytes moved and in what direction. A byte and address counterare a part of the DMA controller 5033. Both are loaded from theprocessor module CPU 5032 to setup the desired DMA transfer. On commandfrom the CPU 5032, a DMA request is made and data is moved from RAMmemory 5040 to the transmit FIFO 5024.

A transfer on the bus 5028 is monitored by each processing module 5034located on the bus 5028. Each bus controller 5022 in the entireprocessor system contains the destination addresses of all devices onthe bus 5028. If a match is found, the input to that receivingprocessing module 5034 FIFO 5026 is enabled. Since multiple messages maybe received by this FIFO 5026, it must have more storage than a transmitFIFO 5024. The receive FIFO 5026 has at a minimum 4 KB×9 of storage.This amount of storage will allow at least 16 messages to queue withinthe receive FIFO 5026 based on the message length of 256 bytes. Amessage burst from multiple sources could conceivably cause multiplemessages to temporarily congest the receive FIFO 5026. The receivingmodule CPU 5032 must have a suitable message throughput from the receiveFIFO 5026 or else a data overflow will result in lost information. DMAis used to automatically transfer messages from the receive FIFO 5026 toRAM 5040. The transfer time from the receive FIFO 5026 to RAM 5040 istypically 21 μsec.

When a message is received by the bus controller 5022, a request for DMAservice is made. Referring to FIG. 59, the DMA controller 5033 generatesa message received hardware interrupt (DMA DONE) and signals processormodule CPU 5032 that it has control of the internal bus 5030. Aninterrupt routine updates the message queue pointer and transfers thecontents of receive FIFO 5026 to RAM memory 5040. The DMA controller5033 is then readied for the next message to be received and points tothe next available message buffer. This continues until all of thecontents of the receive FIFO 5026 are transferred. An end of messagesignal is sent by the receive FIFO 5026 to the DMA controller 5033 viathe bus controller 5022. The processor module 5034 CPU 5032 then regainscontrol of the internal communication bus 5030.

The total elapsed time that it takes for a source to destination messagetransfer is approximately 64 to 85 μsec. As shown in FIG. 60, the timeis computed from when a processor module 5034 starts to send a message,load its transmit FIFO 5024, arbitrate and acquire the data bus 5028,transfer the data to the destination receive FIFO 5126, bus the messageto the CPU 5132 and then finally transfer the message into RAM 5140 ofthe recipient module 5134. The actual throughput is almost 200 timesthat of a 8 KBps time slot on a PCM highway.

Controlling the HSB 5020 requires two state machines; one transmittinginformation 5070, the other receiving information 5072. Both statemachines are implemented in the bus controller 5022 as programmablelogic in the form of Altera's MAX+PLUS II, Version 6.0 state machinesyntax.

Any arbitrary state machine has a set of states and a set of transitionrules for moving between those states at each clock edge. The transitionrules depend both on the present state and on the particular combinationof inputs present at the next clock edge. The Altera EPLD 5022 used inthe preferred embodiment contains enough register bits to represent allpossible states and enough inputs and logic gates to implement thetransition rules.

XXXXV. CDMA Communication System which Selectively Suppresses DataTransmissions During Establishment of a Communication Channel

One of the problems associated with wireless communication of data isthat many different types of communicating nodes are currently in useincluding computers, facsimile machines, automatic calling and answeringequipment and other types of data networks. These nodes may be able tocommunicate at a plurality of different data rates and must be properlysynchronized to avoid losing data during the establishment ormaintenance of a communication.

The present invention includes a feature which prevents the transmissionof data between communicating nodes until the data communication raterequired by the communicating nodes has been completely establishedthroughout the system. The system selectively suppresses theconfirmation tone that a receiving node sends to an originating node.Accordingly, the transmission of voice, facsimile or modem data isprevented until the communication path has been established at thedesired communication rate. This permits the system to reliablytransport encoded data at a plurality of data rates across atelecommunication system which may lack precise synchronization.

Referring to FIG. 61, the communication system 6010 is generallyconnected to originating nodes 6040 and terminating nodes 6044. In orderto conserve as much bandwidth as possible, the communication system 6010selectively allots the bandwidth required for supporting the datatransmission rate required by the originating and terminating nodes6040, 6044. In this manner, the system 6010 ensures that the bandwidthis utilized efficiently. Voiced communications may be effectivelytransmitted across a 32 Kbs ADPCM channel. However, a high speed fax ordata modem signal requires at least a 64 Kbs pulse code modulation (PCM)signal to reliably transmit the communication. Many other types ofmodulation techniques and data transmission rates may also be utilizedby originating and terminating nodes 6040, 6044. The system 6010 must beable to effectively allocate bandwidth and dynamically switch betweenthese data communication rates and modulation schemes on demand.

The communication system 6010 provides a communication link between theoriginating and terminating nodes 6040, 6044. The originating andterminating nodes 6040, 6044 may comprise computers, facsimile machines,automatic calling and answering equipment, data networks or anycombination of this equipment. For robust communication of data it isimperative to ensure that the communication system 6010 switches to thedata transmission rate required by the communicating nodes 6040, 6044prior to the transmission of any data.

Referring to FIG. 62, the typical procedure for establishingcommunications between originating nodes 6040 and terminating nodes isshown. The originating node 6040 periodically transmits a calling tone(step 6100) which indicates that a data communication, (not a voicecommunication), is to be transmitted. The calling tone which is sentfrom the originating node 6040 to the terminating node 6044 is detectedby the terminating node 6044 (step 6102) which initiates severalactions. First, the terminating node 6044 prepares to send a datacommunication (step 6104). Next, the terminating node 6044 transmits ananswering tone (step 6106) to the originating node 6040 to confirm thatthe terminating node 6044 has received the calling tone. Upon receipt ofthe answering tone (step 6108), the originating node 6040 begins thetransmission of data (step 6110), which is received by the terminatingnode 6044 (step 6112). With the communication link established at thedata transmission rate, the originating and terminating 6040, 6044 nodestransmit and receive data until termination of the communication.

One problem with this process is that the transmission rate of thecommunication system 6010 is transparent to both the communicating andterminating nodes 6040, 6044. Modification of the transmission rate froma low rate that supports voice communication to a high rate thatsupports encoded data communication ensures that data will be reliablyand quickly transmitted over a communication channel. However, the newtransmission rate must be completely established throughout thecommunication system 6010 to prevent false interpretation of tonestransmitted by the originating node 6040. The originating node 6040 maybegin transmission of data at a high rate before the system 6010 hasfully switched from 32 Kbs ADPCM to 64 Kbs PCM resulting in loss ofdata.

In order to obviate tone misinterpretation and to prevent the resultingerroneous operation of the originating or transmitting nodes 6040, 6044,the present invention blocks the transmission of the confirming tone tothe originating node 6040 until the new data transmission rate has beencompletely established throughout the communication system 6010. Thisprevents the reception of the answering tone at the transmitting node6040 and ensures the reliable transportation of encoded data at a higherrate across a communication system 6010 which lacks the precisesynchronization which would otherwise be required.

The operation of the system 6010 of the present invention will beexplained with reference to FIG. 63. The communication system 6010facilitates communications between an originating node 6040 and aterminating node 6044. As shown, the actions of the originating node6040 (steps 6202, 6212 and 6214) and the actions of the terminating node6044 (steps 6206, 6207, 6208 and 6218) are the same as in FIG. 62. Theoperation of the communication system 6010 is transparent to both theoriginating node 6040 and the terminating node 6044.

In operation, the originating node 6040 periodically transmits a callingtone (step 6202) which indicates a data communication. The communicationsystem 6010 performs several actions in response to receipt of thecalling tone (step 6204). First, the calling tone is received at 32 KbsADPCM which is the standard communication setting for voicecommunications. The system 6010 detects the calling tone and initiates aswitch to 64 Kbs PCM in order to handle the high-speed datatransmission. This switch must be implemented by the BS 6014, the SU6016 and the controller 6020. Although the system 6010 immediatelybegins the switching over to the new data transmission rate, the processtakes approximately 1500 msec to implement. Accordingly, the system 6010transmits the calling tone to the terminating node 6044 at 32 Kbs ADPCM.

The terminating node 6044 detects the calling tone (step 6206) andprepares to send a data communication (step 6207). The terminating node6044 subsequently transmits the answering tone (step 6208) which, whenreceived by the originating node, will cause the originating node 6040to begin transmission of data.

The communication system 6010 receives the answering tone from theterminating node 6044. However, the system 6010 does not forward theanswering tone to the originating node 6040 until the switch to 64 KbsPCM has been established throughout the system 6010. After the system6010 has confirmed that the switch to 64 Kbs PCM has been achieved, itpermits the answering tone to pass through to the originating node 6040,which receives the tone (step 6212). In response to the answering tone,the originating node 6040 begins transmission of data (step 6214). Thesystem 6010 receives the data and begins transmission of data at the newdata transmission rate of 64 kbs PCM (step 6216) to the terminating node6044 which receives the data (step 6218). Since the communicationchannel has been established, the originating and terminating nodes6040, 6044 continue to communicate over the system 6010 in this manner(steps 6214, 6216 and 6218) until the communication is terminated.

Referring to FIG. 64, a more detailed block diagram of the controller6020 is shown. The controller 6020 controls at least a portion of thecommunication link between two communicating nodes 6040, 6044. This linkcomprises the transmission path 6300 from a first communicating node tothe controller 6020, the transmission path 6302 within the controller6020, and the transmission path 6304 from the controller 6020 to thesecond communicating node. The transmission paths 6300, 6304 to and fromthe controller 6020 may include a plurality of BSs 6014 and SUs 6016which are controlled by the controller 6020.

It should be appreciated by those of skill in the art that theestablishment of a communication channel between communicating nodes6040, 6044 is a complex procedure involving a plurality of tasksperformed by the BS 6014, the SU 6016 and the controller 6020. Adetailed description of the entire procedure is outside the scope of thepresent invention. Accordingly, only those portions of the procedure forestablishment of a communication channel relevant to the presentinvention will be described hereinafter.

The communications between an originating node 6040 and a terminatingnode 6044 are transmitted over a virtual channel as is well known bythose of skill in the art. Since the entire spectrum is used by the CDMAcommunication system 6010, communications from the originating node 6040to the terminating node 6044 are transmitted over the same frequencyband as communications from the terminating node 6044 to the originatingnode 6040. After the virtual channel has been established, theoriginating and terminating nodes 6040, 6044 may freely communicate.

The controller 6020 includes a calling tone detector 6310, amicroprocessor 6312 and an answering tone blocker 6314. The calling tonedetector 6310 monitors the communication channel which has beenestablished in order to detect the calling tone. When a calling tone istransmitted from an originating node 6040, the calling tone detector6310 detects the calling tone, which causes the controller 6020 toinitiate the switch to a higher data transmission rate. Themicroprocessor 6312 subsequently informs any other BSs 6014 or SUs 6016through which the communication is to be routed (hereinafter calledcommunicating equipment) to initiate the switch to the higher datatransmission rate.

The microprocessor 6312 activates the answering tone blocker 6314 whichwill prevent the answering tone from being transmitted through thesystem 6010. Each piece of communicating equipment 6014, 6016, 6020transmits an acknowledgment to the microprocessor 6312 of the controller6020 when the higher data transmission rate has been achieved. Themicroprocessor 6312 subsequently deactivates the answering tone blocker6314 which permits the answering tone to be forwarded to the originatingnode 6040. The communicating nodes 6040, 6044 commence data transmissionover the communication system 6010 at the higher data transmission rate.

Although the invention has been described in part by making detailedreference to the preferred embodiment, such detail is intended to beinstructive rather than restrictive. For example, the functionsperformed by the controller 6020 shown in FIG. 64 may, in an alternativeembodiment, be performed by a BS 6014 coupled with either theoriginating or terminating nodes 6040, 6044. The functions of a BS 6014may also be combined with the controller 6020, to form a master basestation. Additionally, different data rates and modulation schemes maybe employed.

XXXXVI. Efficient Multichannel Filtering for CDMA Modems

Each communication channel within a CDMA communication system typicallyuses DSP (digital signal processing) hardware and software to filter,weight, and combine each signal prior to transmission. The weighting,filtering and combining of multiple signal channels is performed in thetransmit circuitry of a CDMA communication system BS.

Prior art CDMA modems require many multipliers and binary adders forchannel weighting and combining. The filter operation used is equivalentto that of a FIR (finite impulse response or transversal) structure.Each individual FIR filter used also requires many multipliers andadders.

A multiplier implemented in digital form is inefficient and expensive.The expense is directly related to logic gate count. Binary adders areless costly than binary multipliers, however, their use should beminimized. To implement a design using binary multiplication andaddition into an ASIC (application specific integrated circuit) would beexpensive to manufacture and would result in a more inefficient andslower signal throughput.

The disadvantage with prior art CDMA modems is the ability to weight,filter, and combine a plurality of single bit valued signal channelsefficiently and accurately. When a multiplicity of signal processingchannels are involved, the consistency between channels becomesimportant and the cost of hardware per channel escalates. In a CDMAcommunication system, it is necessary to use the minimum amount of powerto achieve the minimum required bit error rate (BER) for maximum usercapacity.

Each channel must have appropriate individual weights applied so thatthe same relative amplitudes are transmitted. After the weightingoperation, each data stream is represented by multibit values. These aretypically summed together in a large digital summing circuit thatconsists of a tree of numerous two input adders. The weighted and summeddigital values are then filtered in a conventional FIR filter. Themultipliers in the FIR process the multibit data and weightingcoefficients to the desired precision. A multichannel filter for a CDMAmodem constructed according to the teachings of the prior art wouldrequire separate FIR integrated circuits rather than total integrationonto an economical ASIC (application specific integrated circuit).

The efficient, multichannel filter for CDMA modems of the presentinvention allows multiple channels consisting of serial, digital bitstreams to be filtered by digital signal processing techniquesperforming sample weighting and summing functions. Each individualchannel may have custom weighting coefficients or weighting coefficientscommon for all channels. If the weighting coefficients are by adaption,the same approach may be taken.

The multichannel FIR filter presented is implemented with no multipliersand a reduction in the number of adders. To increase the speed ofoperation, the filter structure utilizes look-up tables (LUTs) storingthe weighting coefficients. The invention can be constructed either as aFPGA (field programmable gate array) or an ASIC. The use of LUTs savesignificant chip resources and manufacturing costs.

The multichannel FIR filter for CDMA modems in accordance with oneaspect of the present invention is described with reference to thedrawing figures where like numerals represent like elements throughout.Such modems are used in multichannel wireless communication stations inconjunction with the transmission and reception of communicationsignals.

By way of background, many systems have the property of having theiroutputs at a given instant of time depend not only on the input at thetime, but on the entire, or immediate history of the input. Such systemsare said to have memory, averaging past and present samples in arrivingat an output. It is necessary to separate systems with memory into theclasses of discrete and continuous systems. A discrete system is onewhose inputs and outputs are sequences of numerical values rather thancontinuous functions of time.

A sequence of discrete values can be represented as x_(k), where thevalue x is a quantity such as voltage. The subscript k represents thesequence number. Very often in digital signal processing, x_(k)represents a sampled waveform or signal where the subscript specifiesthe point in time at which the sample was taken. However, the subscriptcan represent an alternative meaning such as distance in a spatiallysampled application. For a system to be physically realizable, theoutput must depend only on the present and past history of the input. Noreal system can have an output that depends on the future of the input.The dependence of the output of any physically realizable system on theinput is indicated by:y _(k) =f(x _(k) ,x _(k-1) ,x _(k-2) , . . . ,x _(k-n))  Equation (52)where the input variables are x_(k), the output variable is y_(k), andf(*) is any arbitrary function of n+1 variables. Although this functionis too broadly defined to be analyzed in general, the subset of linearoperations becomes very useful for a plurality of signal processingapplications. These functions also prove to be much more tractable inanalysis.

If the output depends on the previous n samples of the input (a systemhaving a finite memory) in a linear fashion, Equation (52) can bewritten as:

$\begin{matrix}{y_{k} = {{\sum\limits_{j = 0}^{N}\;{a_{j}x_{k - j}}} + b}} & {{Equation}\mspace{11mu}(53)}\end{matrix}$

Such a linear system is characterized by the N+1 weighting variablesa_(j), and by the bias b. An unbiased, discrete linear system ischaracterized by the weighting variables (a₀, a₁, . . . , a_(n)). If theinput x_(k) is a delta function (unity for one sample and zero for allothers), it can be seen that the output of Equation (53) is the sequenceof weighting variables a₀, a₁, . . . , a_(n). Therefore, the response tothe input completely characterizes an unbiased, linear system.

There are certain types of linear systems with memory that can beanalyzed using linear techniques. Even though digital signal processingis discrete by nature, if the input is samples of a continuous input andis sampled sufficiently fast, it is possible to simulate a continuoussystem using the samples as the input variables. The output then appearsas a linear system with a long memory. One such system is a FIR filter7020. A fixed coefficient FIR filter is characterized by theinput/output Equation (54) as follows:

$\begin{matrix}{y_{k} = {\sum\limits_{j = 0}^{N - 1}\;{c_{i}x_{k - j}}}} & {{Equation}\mspace{11mu}(54)}\end{matrix}$as shown in FIG. 65, or expanded asy _(k) =c ₀ x _(k) +c ₁ x _(k-1) + . . . c _(k-1) x _(k-(N-1))  Equation(55)where the FIR filter has an impulse response c₀, c₁, . . . , x_(k)represents the discrete input signal samples at time k; c_(i) are thefilter coefficient weights; N are the number of taps; and y_(k)represents the output at time k. As shown in FIG. 65, the block diagramforms a tapped delay line with the coefficients being known as tapweights.

Digital filters are presently a common requirement for digital signalprocessing systems. In the field of discrete systems, the most populartype of digital filter using convolution is the FIR. FIR filters havetwo advantages. The first is that FIR filters are inherently stable. Thefinite length of the impulse response guarantees that the output will goto zero within N samples. The second advantage is that FIR filters canbe designed and implemented. The FIR filter 7020 can be physicallyrealized by using digital shift registers 7022, multipliers 7024 andsummers 7026 as shown in FIG. 66. The discrete signals 7028 are shiftedinto registers 7022 by a sampling clock pulse 7030. The registers 7022hold past values 7032 of the sampled signal 7028 as well as presentvalues 7034 required for mathematical convolution. The past 7032 andpresent 7034 values are multiplied 7024 by filter weighting coefficients7036, summed 7026 and then output 7038.

Another way of representing a FIR filter structure 7020 is shown in FIG.67. The operation described can be shown to be the equivalent of FIG. 66since:A=_(c) ₃ _(x) _(k-1)   Equation (56)B= _(c) ₃ _(x) _(k-1) +_(c) ₂ _(x) _(k)   Equation (57)C= _(c) ₃ _(x) _(k-2) +_(c) ₂ _(x) _(k-1)   Equation (58)resulting in

$\begin{matrix}{D = {y_{k} = {{{c_{3}x_{k - 3}} + {c_{2}x_{k - 2}} + {c_{1}x_{k - 1}} + {c_{0}x_{k}}}\mspace{79mu} = {{\sum\limits_{j = 0}^{3}\;{c_{j}x_{k - j}}}\mspace{79mu} = {c_{k}*x_{k}}}}}} & {{Equation}\mspace{11mu}(59)}\end{matrix}$As can be seen in FIGS. 66 and 67, the weighting 7036 of the discreteinput samples 7028 relies upon many multipliers 7024.

A single channel of a multichannel FIR filter 7040 for CDMA modems isshown in simplified form in FIG. 68A. The multichannel FIR filter 7040is shown as a single element with a multichannel input sequence x_((i)k)entering the filter 7040 and the filtered result y k exiting. Thesubscript “i” identifies which channel from a plurality of channels isbeing filtered. The multiple single bit data/signal streams representserial data streams that have been modulated with a pseudo noise (PN)spreading code. Each channel could represent user traffic channels atvarious data rates. Various types of signaling data might comprise otherchannels.

A typical example of an integrated service digital network (ISDN) CDMAmodem would require five channels. Two channels would be 64 kbps trafficchannels (B1 and B2), a 16 kbps auxiliary signaling and packet channel(D), an order wire channel (OW), and an ARPC channel.

For maximum user capacity in a CDMA system it is necessary to use theminimum amount of power to achieve the required BER. Each channel musthave the appropriate individual weight applied so that the correctrelative amplitudes are transmitted. After the weighting operation theindividual data streams become multibit values. The data streams aresummed together in a large digital summing circuit that consists of atree of numerous two input adders. The weighted and summed digitalvalues are then filtered in a conventional FIR filter. The FIR filter isrequired to pulse shape the input waveforms while suppressingout-of-band emissions. The multipliers in the FIR must handle themultibit data and coefficients to the desired precision.

In FIG. 68B, four signal channels are input individually into separateFIR filters 7020, (the clock signal has been omitted for clarity). Theindividually filtered signals are then weighted using multipliers 7024with a channel specific weighting coefficient 7037 w_((i)) for powercontrol, equalizing the power or gain between individual channels,before being input to a multichannel summer 7046. Since all users occupythe same frequency spectrum and time allocation in spread spectrumcommunication systems, it is desired that each user is received with thesame power level. The result, y_((i)k) 7044, is a weighted sum of theindividually FIR filtered multiple signal channels.

A CDMA transmitter combines many channels of varying types of digitalsignals (serial digital voice, power control, ISDN data). Typically,each channel is modulated with a different spreading code. The spreadingcode allows a CDMA receiver to recover the combined signals by use ofthe proper code during demodulation. Alternatively, any set oforthogonal functions could be combined with the preferred embodiment andlater separated by correlation.

The output 7044 of the multichannel FIR filter 7040 is a weighted andfiltered average. Although each channel has been described as a singlebit valued serial data stream, multi-bit values or levels may beprocessed with the identical multichannel filter structure.

Referencing FIG. 69, the multichannel FIR filter 7040 is shown usingfour tap FIR filters 7048. The weighting of the discrete samples isperformed by conventional multipliers 7024. Each FIR structure iscomprised of shift registers 7022 and summers 7026 for past 7032 andpresent 7034 sampled signals. Each tap weight coefficient 7036 ismultiplied by the respective channel power control weighting factor7037. The result is the same as shown in FIG. 68B, but with the externalmultipliers inside the FIR 7048 structures.

Hardware reduction is accomplished by sharing FIR registers and addersas shown in FIG. 70. Each multichannel processing element 7052 performspart of the channel weighting 7037, the FIR tap coefficient 7036multiply 7024, and the summing 7026 of the multiple channels for thattap. The partitioning of the discrete functions reveals the preferredembodiment.

FIG. 71 shows the multichannel processing element 7052 as a processingblock with “N” single bit input signals x_((o)k), x_((1)k), . . . ,x_((N)k). The computed output z_(k) 7054 contains “W” bits ofresolution. The discrete input signals 7028 form a vector. This vectorcan be assigned an overall value by weighting each bit with anincreasing power of two. In the alternative, the multichannel signalbits are treated as a binary valued word. The output of the processingblock is a “W” bit wide function of the N bit binary input argument. Theblock performs the equivalent logical function of a memory device wherethe input signal bits form an address and the computed values arecontents of the selected memory word. A memory based LUT 7056 canperform an arbitrary function quickly and efficiently as shown in FIG.72A.

A mathematical function ƒ of an argument x with a result of y isexpressed as y=ƒ(x). The function performs a mapping of all values of xinto another space of y values. A LUT performs this mapping for thevalues of interest in the preferred embodiment. The LUT memory device ispresented with an address of a location within the memory circuit. Thevalue previously stored at that location is delivered to the memoryoutput data bus. The values of interest of x, which are discrete, aremapped into a binary number. Since the multichannel signals arerepresented by zero or one logic levels, they are used as bits to form abinary number. Every possible combination of channel values is thereforeassigned a state number. This operation is represented as:

$\begin{matrix}\begin{matrix}{{\sum\limits_{j = 0}^{M - 1}\;{x_{j}2^{j}}} = {{x_{M - 1}2^{M - 1}} + \mspace{11mu}{\ldots\mspace{14mu} x_{3}2^{3}} + {x_{2}2^{2}} + {x_{1}2^{1}} +}} \\{= {{x_{M - 1}2^{M - 1}} + \mspace{11mu}{\ldots\mspace{14mu} x_{3}8} + {x_{2}4} + {x_{1}2} + x_{0}}}\end{matrix} & {{Equation}\mspace{11mu}(60)}\end{matrix}$

Each state is a binary number that references an address in the LUT. Theoutput value from the LUT is the precomputed value of the functionresultant that would occur given the argument corresponding to thataddress. This is illustrated as a tabular representation of the LUTcontents. The function to be performed is the weighted sum of themultiple channels for a given single tap of the FIR structure.

For example, in an application using 4 channels (M=4), the LUT contentslocated at the 2nd tap of the multichannel FIR (j=2) would be as shownin Table 16.

TABLE 16 Values of x LUT Value Stored At x₃, x₂, x₁, x₀ AddressComputation of A Location A 0 0 0 0 0 0 0 0 0 1 1 = 1 w₀c₂ 0 0 1 0 2 = 2w₁c₂ 0 0 1 1 2 + 1 = 3 w₁c₂ + w₀c₂ 0 1 0 0 4 = 4 w₂c₂ 0 1 0 1 4 + 1 = 5w₂c₂ + w₀c₂ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 1 1 0 1 8 + 4 + 1 = 13 w₃c₂ + w₂c₂ + w₀c₂1 1 1 0 8 + 4 + 2 = 14 w₃c₂ + w₂c₂ + w₁c₂ 1 1 1 1 8 + 4 + 2 + 1 = 15w₃c₂ + w₂c₂ + w₁c₂ + w₀c₂

The LUT 7056 memory words contain precomputed values corresponding tothe current input address value as shown in FIG. 72B. The memory can beimplemented in either ROM or RAM, depending upon the application.

In the preferred embodiment, ROM (read only memory) is used to storepermanent LUT values. This is implemented efficiently as an integratedcircuit. ROM is appropriate for time invariant systems where therequired channel weights and filter coefficients are known a priori. RAM(random access memory) allows new values to be written over old. LUTvalues can be computed and loaded to achieve adaptivity. RAM is not asspace efficient as ROM but is still efficient considering the increasedflexibility.

The preferred embodiment of the multichannel FIR filter 7040 for CDMAmodems according to the present invention is shown in FIG. 73. Thefilter structure uses LUTs 7056 rather than the inefficient multichannelprocessing elements 7052 which require a plurality of multipliers 7024and summers 7026.

The signal bits form the address word which is applied to the LUT 7056.There is a LUT 7056 for each filter tap required. The contents of eachLUT 7056 is computed as:

$\begin{matrix}{{L_{j}( {D_{N},{D_{{N - 1}\mspace{11mu}}\ldots}\mspace{11mu},D_{2},D_{1}} )} = {C_{j}{\sum\limits_{i = 1}^{N}\;{D_{i}W_{i}}}}} & {{Equation}\mspace{11mu}(61)}\end{matrix}$

As shown, any combination of signal values has its weighted sumprecomputed. The multiplication of each tap coefficient of the FIRfunction is included in the precomputed table.

The weighted and filtered single channel operation of FIG. 68A with an Ntap FIR can be expressed as

$\begin{matrix}{y_{{(i)}k} = {{w_{i}{\sum\limits_{j = 0}^{N - 1}\;{c_{{(i)}j}x_{{{(i)}k} - j}}}} = {w_{i}\lbrack {c_{{(i)}j}*x_{{(i)}j}} \rbrack}}} & {{Equation}\mspace{11mu}(62)}\end{matrix}$An M channel multichannel version of this is shown in FIG. 68B and canbe expressed as

$\begin{matrix}{y_{{(i)}k} = {{\sum\limits_{i = 0}^{M - 1}\; y_{{(i)}k}} = {\sum\limits_{i = 0}^{M - 1}( {w_{i}{\sum\limits_{i = 0}^{M - 1}\;{c_{{(i)}j}x_{{{(i)}k} - j}}}} )}}} & {{Equation}\mspace{11mu}(63)} \\{y_{{(i)}k} = {\sum\limits_{i = 0}^{M - 1}{w_{i}\lbrack {c_{{(i)}j}*x_{{(i)}j}} \rbrack}}} & {{Equation}\mspace{11mu}(64)}\end{matrix}$This is the desired weighted sum of convolutions or FIR filteringoperations. The convolution is performed in FIR filters 7020, theweighting in multipliers 7024 and the summation in adders 7046. Theconvolution achieved is identical to that originally presented inEquation (64). The summation and weights are a result of the extensionto a multichannel process.

The preferred embodiment shows an improved filter for multichannel CDMAFIR filtering modem applications. It has been shown that the signalprocessing operation over multiple channels, as shown in FIGS. 68A and68B, can be implemented using no multipliers and a reduced number ofadders.

While the present invention has been described in terms of the preferredembodiment, other variations which are within the scope of the inventionas outlined in the claims below will be apparent to those skilled in theart.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base stationincludes but is not limited to a Node-B, site controller, access pointor any other type of interfacing device in a wireless environment.

1. A transmission circuit configured to spread code a first data and asecond data in accordance with two different corresponding spreadingcodes, a first spreading code and a second spreading code; thetransmission circuit further being configured to output the spread firstdata using a first spreading code on a first channel and spread seconddata using a second spreading code on a second channel, respectively,the first channel derived from the first data using the first spreadingcode at a first data rate, and the second channel derived from thesecond data using the second spreading code at a second data rate, thefirst and second data rates being different from one another, whereinthe transmission circuit is further configured to output a thirdchannel, which does not carry the first and second data, the thirdchannel derived by spread coding a power control bit and signalinginformation with a corresponding spreading code, the signalinginformation facilitating the processing of radio frequency signals by abase station, wherein the transmission circuit is further configured tocombine the first channel, the second channel and the third channel; andan antenna coupled to the transmission circuit, the antenna outputting aradio frequency signal derived from the combined the first, the secondand the third channels; wherein the first channel, the second channeland the third channel are transmitted at the same time, wherein thetransmission circuit is further configured to determine the powercontrol bit based on a power level of a channel received by the antennaand an interference or a noise level associated with the receivedchannel.
 2. The code division multiple access subscriber unit inaccordance with claim 1, wherein the power control bit is one of aplurality of power control bits, and wherein the transmission circuit isfurther configured to time division multiplex the plurality of powercontrol bits with the signaling information onto the third channel. 3.The code division multiple access subscriber unit in accordance withclaim 1, wherein the power control bit is one of a plurality of powercontrol bits that are not error protected.
 4. The code division multipleaccess subscriber unit in accordance with claim 1, wherein the powercontrol bit is one of a plurality of power control bits that are notsubject to forward error correction encoding.
 5. The code divisionmultiple access subscriber unit in accordance with claim 1, wherein thesubscriber unit further comprises: a first interface coupled to thetransmission circuit; and a second interface coupled to the transmissioncircuit, the first interface receiving the first data and the secondinterface receiving the second data.
 6. A communication method for usein a code division multiple access (CDMA) subscriber unit, comprising:producing a first data and a second data; spread coding the first dataand the second data in accordance with two different correspondingspreading codes, a first spreading code and a second spreading code;outputting the spread coded first data using a first spreading code on afirst channel and second data using a second spreading code on a secondchannel, respectively; spread coding combining a power control bit andsignaling information with a corresponding spreading code to produce athird channel, such that the third channel does not carry the first andsecond data, the signaling information facilitating processing of radiofrequency signals by a base station; combining the first channel, thesecond channel, and the third channel; and transmitting, with an antennaof the code division multiple access subscriber unit, a radio frequencysignal that is derived from the combined the first, second and thirdchannels, such that a first data rate associated with the first datacarried by the first channel is different than a second data rateassociated with the second data carried by the second channel, whereinthe first, second and third channels are transmitted at the same time,wherein the transmission circuit is further configured to determine thepower control bit based on a power level of a channel received by theantenna and an interference or a noise level associated with thereceived channel.
 7. The communication method in accordance with claim6, wherein the power control bit is one of a plurality of power controlbits, and the combining includes time division multiplexing theplurality of power control bits with the signaling information onto thesecond channel.
 8. The communication method in accordance with claim 6,wherein the power control bit is one of a plurality of power controlbits that are not error protected.
 9. The communication method inaccordance with claim 6, wherein the power control bit is one of aplurality of power control bits that are not subject to forward errorcorrection encoding.
 10. The communication method in accordance withclaim 6, further comprising receiving the first data through a firstinterface of the code division multiple access subscriber unit andreceiving the second data through a second interface of the codedivision multiple access subscriber unit, the first data including firstdigital data and the second data including second digital data.
 11. Thecommunication method in accordance with claim 10, wherein the firstdigital data includes a plurality of frames.
 12. A code divisionmultiple access subscriber unit, comprising: a transmission circuitconfigured to spread code a first data and a second data in accordancewith two different corresponding spreading codes, a first spreading codeand a second spreading code, the transmission circuit further beingconfigured to output the spread first data using a first spreading codeon a first channel and spread second data using a second spreading codeon a second channel, respectively, the first channel derived from thefirst data using the first spreading code at a first data rate, and thesecond channel derived from the second data using the spreading code ata second data rate, the first and second data rates being different fromone another, wherein the transmission circuit is further configured tooutput a third channel that does not carry the first and second data,the third channel including derived by spread coding a power control bitand signaling information with a corresponding spreading code, thesignaling information facilitating the processing of radio frequencysignals by a base station, wherein the transmission circuit is furtherconfigured to combine the first channel, the second channel and thethird channel; and an antenna coupled to the transmission circuit, andconfigured to transmit a radio frequency signal derived from thecombined the first, second and third channels, wherein the first, thesecond and the third channels are transmitted at the same time, whereinthe transmission circuit is further configured to determine the powercontrol bit based on a power level of a channel received by the antennaand an interference or a noise level associated with the receivedchannel.
 13. The code division multiple access subscriber unit inaccordance with claim 12, wherein the power control bit is one of aplurality of power control bits, and wherein the transmission circuit isfurther configured to time division multiplex the plurality of powercontrol bits with the signaling information onto the third channel. 14.The code division multiple access subscriber unit in accordance withclaim 12, wherein the power control bit is one of a plurality of powercontrol bits that are not error protected.
 15. The code divisionmultiple access subscriber unit in accordance with claim 12, wherein thepower control bit is one of a plurality of power control bits that arenot subject to forward error correction encoding.
 16. The code divisionmultiple access subscriber unit in accordance with claim 12, wherein thesubscriber unit further includes: a first interface coupled to thetransmission circuit; and a second interface coupled to the transmissioncircuit, the first interface receiving the first data and the secondinterface receiving the second data that comprise first and seconddigital data, respectively.
 17. The code division multiple accesssubscriber unit in accordance with claim 16, wherein the first digitaldata includes a plurality of frames.